Add System.Composition source code
Fix build errors caused by moving using statements outside of namespaces
Move System.Composition source to corefx directory structure
Add solution for System.Composition, update source projects to build.
Remove unneeded System.Composition files
Rename System.Composition "test" folder to "tests"
Add test projects to System.Composition solution
Remove custom targets for System.Composition tests.  Add project dependency of tests to solution.
Remove "Desktop" name from System.Composition test projects
Reference xunit from System.Composition test projects
Convert tests to xUnit