Add support for running Jetpack from a symlinked folder

I thought it was about time I developed a fix for the staticize subdomain problem #1918 so I synced the latest version and created a Windows symlink to it
`C:\apache\htdocs\wporg\wp-content\plugins>mklink /J jetpack c:\github\jetpack`

It didn't look too promising.


As you can see from the screen capture, there are a number of files with bad URLs
e.g.

```
http://qw/wporg/wp-content/plugins/C:/github/jetpack/css/jetpack-icons.min.css?ver=3.5
```

It would be nice if I could run Jetpack directly from a symlinked folder.

Workaround: Install it as a normal plugin.

No source files or tests are named. Reproduce the Jetpack installation from a Windows symlink and trace how the plugin builds static asset URLs, using the malformed C:/github/jetpack path as the starting case. Done means symlinked installations produce valid asset URLs while normal plugin installations continue to work.

Written by the indexing model from the issue text.