Move xunit.abstractions over to the new shared project system.
Added Assert.Matches and Assert.DoesNotMatch assertions. #32
Added set comparison assertions. Closes #56
Report xunit1 exceptions in class start/finish
Nested exceptions in xunit1 class start/finish
Nested exceptions in xunit1 tests
Nested exceptions in xunit1 test failure

This tests exceptions reported as a failure, rather than last commit,
which is RunTests throwing exceptions
xunit1 sends method start/finish messages
Call testStart messages for xunit1 skipped tests
Tests in the same class executed in parallel (closes #68)
Discovery process throws exception when CollectionBehaviour is set to PerClass (fixes #67)
Extensibility point overhaul: XunitTestFramework, XunitTestFrameworkDiscoverer, XunitTestFrameworkExecutor.