Unity executable path option added

With the frequent release of new versions of Unity, it is sometimes useful to run builds against different versions.
Adding an option to choose the path of the executable allows users to install multiple versions of Unity on their agents and to choose the appropriate version to run their builds. 
Default behaviour is left unchanged : if the new field is left blank, Unity executable is still searched in default directories.

Screenshot :
![screen shot 2014-12-12 at 12 55 35 pm](https://cloud.githubusercontent.com/assets/4907499/5416528/528bf9da-81fe-11e4-8e66-b2d0d7f4c205.png)