Upgrade tests to use Xunit 2.0
Fix DEBUG constant name
Fix bug in Matrix4x4.Decompose.

While working on the Win2D numerics implementation, I discovered this longstanding issue
which was inherited from XNA. In some corner cases where one axis of a matrix transform
has degenerate (zero) scale factor, the Decompose method tries to compensate by filling
in an artificially generated orthogonal coordinate system, but was doing this wrong and
generating nonsense results.

Also updated unit tests to properly validate decompose of matrices with zero and negative
scale factors.
Fix some more XML docs
Improve Dictionary access
Merge pull request #250 from shawnhar/matrixdecompose

Fix bug in Matrix4x4.Decompose.
Formatting changes to private fields.
Merge pull request #256 from Priya91/lasekar/netformat

Formatting changes to private fields.
Remove app.config from Registry.
Merge pull request #254 from alfaproject/regex_dictionary

Improve Dictionary access
Prepare metadata reader tests to be portable
Build metadata reader tests as PCL
Re-enable some Vector tests which were flaky on CI machines
Merge pull request #259 from Priya91/registry

Remove app.config from Registry.
Merge pull request #263 from mellinoe/enable-vector-tests

Re-enable some Vector tests which were flaky on CI machines
Optimize ternary condition return
Merge pull request #268 from alfaproject/regex_ccek_equals

Clean up a line to avoid a ternary operator
Add System.IO.Pipes source code
Merge branch 'xunit2' into test_on_coreclr

Conflicts:
	src/System.Numerics.Vectors/tests/Matrix3x2Tests.cs
	src/System.Numerics.Vectors/tests/Matrix4x4Tests.cs
	src/System.Numerics.Vectors/tests/PlaneTests.cs
	src/System.Numerics.Vectors/tests/QuaternionTests.cs
	src/System.Numerics.Vectors/tests/Vector2Tests.cs
	src/System.Numerics.Vectors/tests/Vector4Tests.cs
	src/System.Reflection.Metadata/tests/TestUtilities/WorkItemAttribute.cs
Merge branch 'mdreader-portable-tests' into test_on_coreclr

Conflicts:
	src/System.Reflection.Metadata/tests/Metadata/MetadataReaderTests.cs
	src/System.Reflection.Metadata/tests/System.Reflection.Metadata.Tests.csproj
	src/System.Reflection.Metadata/tests/packages.config