Various Updates to get the Content Pipleine working on Mac
Slight changes to the API to allow custom Loggers to be assigned,
This is useful for the Addins and any custom UI.
Changed the ContentTypeWriter to emit XNA style assembly
references for backward compatability (for now)
Updated the SpriteFont processor to attempt to write
compressed textures
Added Core.Graphics support for the FontHelper (thanks kjpou1)
Merge remote-tracking branch 'upstream/develop' into pipeline

Conflicts:
	MonoGame.Framework.Content.Pipeline/Builder/PipelineManager.cs
	MonoGame.Framework.Content.Pipeline/Graphics/FontHelper.cs
	MonoGame.Framework.Content.Pipeline/Graphics/GraphicsUtil.cs
	MonoGame.Framework.Content.Pipeline/MonoGame.Framework.Content.Pipeline.MacOS.csproj
	MonoGame.Framework.Content.Pipeline/Processors/TextureProcessor.cs
Fixed a few issues with the merge
Some changs to the Mac and Linux projects to fix up some of the dependencies
Fixed the FontDescriptionProcessor.cs
Bump ThirdParty/Libs to include the .dll.config for nvtt
Fixed some paths for the Mac dependecies
Reverted previous changes to ContentTypeWrtier.
Fixed an issue in the project file
Updated FontDescriptionProcessor to use SharpFont
Added SharpFont to the content Projects
Updated Submodule to include the new SharpFont assembly
Added Content Piplein Mac Solution
Merge remote-tracking branch 'upstream/develop' into pipeline

Conflicts:
	Samples
Bump ThirdParty/Libs to bring in nvtt linux binaries
Fixed up the dependencies for the platforms
Merge remote-tracking branch 'upstream/develop' into pipeline
Merge pull request #1900 from dellis1972/pipeline

Content Pipeline fixes for MacOS and Linux
Merge branch 'develop' of git://github.com/mono/MonoGame into monogame-sdl2