Merge pull request #1546 from AdamDawes/FixForMatrixCreateBillboard

Fixed the Matrix.CreateBillboard function so that both overloads work.
We set the SamplerState even if the texture is null.
Disabled RenderTargetBinding to RenderTarget3D for non DX builds.
Added Leaderboard classes to Android project. Added missing LeaderboardWriter property from Gamer class.
Fixed memory leak on resume.
Merge branch 'develop' of https://github.com/mono/MonoGame into develop
Added Missing Leaderboard classes to other all platforms.
Added missing Lidgren reference, no idea how it lost the reference.
Changes to allow Windows 8 to build with GamerServices changes.
Added Missing Exception definition to project.
Small performance improvement, no need to create layouts on every draw.
Implemented missing SoundEffect.IsDisposed
Merge pull request #1547 from theodis/develop

Fixed missing keyboard mapping for Linux
Merge pull request #1560 from dellis1972/soundeffectdisposed

Implemented missing SoundEffect.IsDisposed
AccelerometerAvailable will never return true when running in the simulator. Added check if running on simulator to return true.
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.
Re-orginization of the Test folder.
The tests are now under Framework folder.
The test runner support classes are under the Runner folder.
Removed Android Application tag, which creeped in :(