Add PlasticSCM plugin code that will ship in Unity side (which only chec...

Add PlasticSCM plugin code that will ship in Unity side (which only checks whether there is a valid PlasticSCM's Unity installation, and if so, spawns the real plugin app).
Windows wrapper is a .net c# 2.0 app. MacOS wrapper is a bash script.

Created a new branch whose base is the commit "5d0625c4f1e529472ddf0fb5fe7be49f48759df1" located in "unity4.3" branch.