v2.2.0-a1 (VS2010 and WP7 compatible, not only VS2012 and VS2013)

- Supporting .Net451, .Net452, SL40WP70 and SL40WP71 projects
- Default TargetFrameworkVersion is 4.0 to properly support VS2010
- supporting PreBuildEvent and PostBuildEvent #39 (transform on build is
removed)
- .nuspec templates are UTF-8 #52

- VSSDK.Shell.10 NuGet package is added to Package.csproj to properly
build for VS2010 under VS2012
- NuGet.Core updated from 2.8.0 to 2.8.1
- Proper handling of OutputPath and OutDir properties in NuBuild.targets
- detailed error messages during build

Issues:
- serialization exception during VS2010 build
- automatic dependency calculation screws up with mixed SL40WP7x project
types