Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
Add optimization for ImmutableList<T>.AddRange when adding non-empty range
to non-empty list
check for null parameter as base class does, but throw ArgumentException
Apply same optimization for ImmutableList<T>.InsertRange, also add more
  test coverage
XmlAttributeCollection.SetNamedItem(XmlNode) returns null if provided a null parameter
Remove unused using statements
Update Developers.md

Fixing the name of and adding a link to Visual Studio Community 2013.
Merge remote-tracking branch 'upstream/master' for SetNamedItemTests
Sort usings.
Merge branch 'remove-unused-usings'
Fix build break due to using being removed that was actually used.
Documentation comments: CDATA to {T} for generics

Replaced CDATA with {T} syntax for generics in documentation comments.
Changed references of (float)Math.PI to use its own Pi property instead.
Fix System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() to not generate Comparer wrapper each time
System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() (3 overloads) creates a new Comparer wrapper each time. This can get expensive if you call Sort a lot.

This fix optmizes method Sort() alone among the 3 overloads. Sort() uses the default comparer which through this fix is getting cached into a static field in type Comparer such that the Comparer wrapper is not created each time Sort() is called.
Existing testcase Sort(), covers this case.
Make InsertRange rebalance list correctly
Change Node.Root to internal to facilitate testing
Revert changes to InsertRangeTest
Add more tests
Merge branch 'master' of https://github.com/arecker/corefx into fix-math-pi
Clean up XML code
Fixing System.Xml.XPath.XDocument implementation
Update Developers.md

Visual Studio Community and Visual Studio 2015 Preview text and links
Merge branch 'xml-check-not-null' of https://github.com/jltrem/corefx into jltrem-xml-check-not-null