Fixes to Game startup when using a unit test runner.
The Content.RootDirectory cannot be absolute.
Removed local directory reference.
Using AppDomain.CurrentDomain.BaseDirectory seems to be a more reliable way of getting the working dir.
I was wrong.... Content.RootDirectory can have an absolute path.  MonoGame needs fixing here.
Game takes care of disposing GraphicsDeviceManager.
We set the SamplerState even if the texture is null.
Implemented missing SoundEffect.IsDisposed
Merge pull request #1560 from dellis1972/soundeffectdisposed

Implemented missing SoundEffect.IsDisposed
Addition of the MathHelper unit test. More tests are needed.
Merge pull request #1530 from SickheadGames/testfix

Unit Test Fixes
Merge pull request #1562 from totallyeviljake/MATH_HELPER_UNIT_TEST

Addition of the MathHelper unit test. More tests are needed.
Merge pull request #1548 from SickheadGames/effectpass

Fix SamplerState in EffectPass