Added frameworkAssemblies support
- The frameworkAssemblies section is specific to a target framework
- Refactored BuildContext to call into the ProjectReferenceDependencyProvider.
This means that only a single place actually uses the project class directly
(which is nice).
- GetLibraryDescription now takes a Library instead of name and version
(we may need to revisit this and create a different poco). When @lodejard finishes
dependency types we'll see how this ends up
- Preserve the backwards compatibility of declaring frameworkAssemblies in
dependencies temporarily
- Added tests for parsing frameworkAssemblies

#640
#619
Assume any reference resolved by the framework resolver is a framework ref
Skip framework assemblies for aspnetcore50
Fixup the dependency version when resolving GAC references from the project.
Remove wildcard from the global.json lookup
Reduce number of files in packed output

- Nupkgs, nuspecs and SHA files are not copied to output packages folder
- Only copy assemblies needed by the framework specified with '--runtime'