OSX Installation Error: Package 'libunwind' not found

While it's related to #96 in the sort of error it is, `libzmq` is found when installed with Homebrew via `brew install zmq`. However, `go get` still fails due to requirement of `libunwind` in `libzmq.pc` file that is provided by ZeroMQ installation.

Error seen below:
```
go get -u github.com/gopherdata/gophernotes
# pkg-config --cflags -- libzmq
Package libunwind was not found in the pkg-config search path.
Perhaps you should add the directory containing `libunwind.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libunwind', required by 'libzmq', not found
pkg-config: exit status 1
```