MathHelper - added comments

added comments and added method overload of Clamp
fix new method in mathHelper
Merge pull request #1276 from slygamer/pipeline

Add built-in System and Math content type writers
Merge pull request #1277 from Shqrdx/develop3d

Test commit for MonoGameTeam. Added comments to MathHelper.
Added some tests to check for SDK's before building
Fixed issue with build script
removed call to installer as that will be a seperate task
Merge branch 'develop3d' of https://github.com/mono/MonoGame into pipeline