Added convenience which removes the need to copy .dll's to unity's application folder. 

Using the strategy as described here http://forum.unity3d.com/threads/31083-DllNotFoundException-when-depend-on-another-dll/page2 , PluginUtils.cs dynamically appends the plugins folder to the environment PATH variable. The circumvents the need to copy anything to the Unity.exe application folder - which is pretty gross. 

Tested on Win and OSX.