Added experimental function arguments to the vim completer.

Added some code to display the first letter of the completion type, the arguments for function, and function return types.

I'm not a vimscript expert so it may not be perfect. Set g:racer_experimental_completer to 1 to enable it.

![racer](https://cloud.githubusercontent.com/assets/7722833/3822152/e050270a-1d1d-11e4-9601-3a44c3d5ea7e.png)