Added Artemis_UnitTests.csproj into ArtemisEntitySystemPcVS2012.sln

Added UnitTests project into PcVS2012 solution.
At least to conform with the Readme and PcVS2010 solution:
"Use ArtemisEntitySystemPcVS2012 to develop on Entity System and
Unit-tests only."
Merge pull request #9 from thelinuxlich/master

sync with master repo