Rake task for Windows installations (fix for issue #298)

I added a rake task called ```windows``` which adds the hub bin directory to the windows local user ```PATH``` variable, and also creates a file called ```hub.bat``` inside the bin directory. This essentially automates the steps I listed in [issue #298](https://github.com/github/hub/issues/298 "Issue #298").

The reason I put the existing ```install``` code inside the ```else``` check is because there is no environment variable called prefix in windows, nor is there a ```/usr/local```. If there's some other way to install ```man``` pages in windows they can be put outside of the ```else```.