Add test coverage for XPath and XDocument
Clean up Xml directory structure
Rename binary of System.Xml.XPath.XDocument tests
Update dependency to Microsoft.DotNet.BuildTools 1.0.13-prerelease

Build tools 1.0.13 has Nick's commit b7c62ee.. which builds each
assembly in to its own intermediate directory.  This is required
to fix a bug where XML tests would fail if they were rebuilt without
the workspace being cleaned because they would pick up incorrect
versions of resource files.