Add cocoapods support

Add cocoapods support for easier dependency management in Xcode.

My `Podfile` is currently: 

```
pod 'BMGlyphLabel', :git => 'https://github.com/xinsight/BMGlyphLabel.git'
```

Once you have the `.podspec` in your repo, and you register your with cocoapods, the `:git` attribute is not needed.

See http://cocoapods.org/ for more info.