npm install on Mac OSX El Capitan

I was following the Mac OSX install guide from the wiki. It's pretty good, but I ran into an error: pkg-config not found.

```
$ npm install

> node-protobuf@1.4.1 install /Users/vkwok/workspace/POGOserver/node_modules/node-protobuf
> node-gyp rebuild

lp.sh: line 2: pkg-config: command not found
gyp: Call to 'sh lp.sh' returned exit status 0 while in binding.gyp. while trying to load binding.gyp
gyp ERR! configure error
gyp ERR! stack Error: `gyp` failed with exit code: 1
gyp ERR! stack at ChildProcess.onCpExit (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:305:16)
gyp ERR! stack at emitTwo (events.js:106:13)
gyp ERR! stack at ChildProcess.emit (events.js:191:7)
gyp ERR! stack at Process.ChildProcess._handle.onexit (internal/child_process.js:215:12)
gyp ERR! System Darwin 15.6.0
gyp ERR! command "/usr/local/bin/node" "/usr/local/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
gyp ERR! cwd /Users/vkwok/workspace/POGOserver/node_modules/node-protobuf
gyp ERR! node -v v6.7.0
gyp ERR! node-gyp -v v3.3.1
gyp ERR! not ok
npm WARN POGOServer@0.6.1 license should be a valid SPDX license expression
npm ERR! Darwin 15.6.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install"
npm ERR! node v6.7.0
npm ERR! npm v3.10.3
npm ERR! code ELIFECYCLE

npm ERR! node-protobuf@1.4.1 install: `node-gyp rebuild`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the node-protobuf@1.4.1 install script 'node-gyp rebuild'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the node-protobuf package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR! node-gyp rebuild
npm ERR! You can get information on how to open an issue for this project with:
npm ERR! npm bugs node-protobuf
npm ERR! Or if that isn't available, you can get their info via:
npm ERR! npm owner ls node-protobuf
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR! /Users/vkwok/workspace/POGOserver/npm-debug.log
```

I fixed this by installing pkg-config:

```
$ curl https://pkg-config.freedesktop.org/releases/pkg-config-0.28.tar.gz -o pkgconfig.tgz
$ tar -zxf pkgconfig.tgz && cd pkg-config-0.28
$ ./configure && make install
```

If there's an error like this:

```
configure: error: Either a previously installed pkg-config or "glib-2.0 >= 2.16" could not be found. Please set GLIB_CFLAGS and GLIB_LIBS to the correct values or pass --with-internal-glib to configure to use the bundled copy.
```

Then run `./configure --with-internal-glib && make install`

Not sure if any of you also ran into this, but I thought it'd be nice to add to the existing wiki.

Start with the Mac OSX install guide in the wiki and review the reported npm install output involving node-protobuf and pkg-config. Update the guide with the pkg-config installation commands and the --with-internal-glib fallback, then verify that the documented steps address the reported error.

Written by the indexing model from the issue text.