Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
check for null parameter as base class does, but throw ArgumentException
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.
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
Update Developers.md

Add text and links for Visual Studio Community and Visual Studio 2015 Preview.
Merge branch 'patch-1' of https://github.com/jac009/corefx
Add license header to test source files.