Add a configuration setting for a PATH prefix

Related to #5 and #93. Adds ability to "shim" the path by prepending a configuration setting if available.

Configuration:
![configuration](https://cloud.githubusercontent.com/assets/1694055/2688554/4d171f46-c2ac-11e3-85e9-71fef5ba6e34.png)

Running Ruby with and without a path prefix:
![path_shim](https://cloud.githubusercontent.com/assets/1694055/2688555/4ecad2ce-c2ac-11e3-986e-15492335216a.gif)