Add outer-loop tests support

To create outer-loop test use [OuterLoop] attribute.
To mark failing test use [ActiveIssue(issueNumber)].

This PR fixes following issue:
https://github.com/dotnet/corefx/issues/18

To run all tests except failing and outer-loop:
build

To run all tests except failing:
build /p:ExcludeTraits=category=failing

To run all tests related with issue X:
build /p:ExcludeTraits=None=None /p:IncludeTraits=ActiveIssue=X

To run only non-failing outer-loop tests:
build /p:ExcludeTraits=category=failing /p:IncludeTraits=category=outerloop

To run all tests:
build /p:ExcludeTraits=none=none