Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
Fix typos in XML comments
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
System.Xml.XmlDocument.Tests formatter changes
System.Xml.XmlDocument.Tests Sanitizer changes
Replacing foo:bar with val1:val2
Restoring TODOs in comments
Merge branch 'xmltypo'
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.