Remove unused usings (Targeting System.Linq)
Replace Linq usages with non-linq implementations.
Remove Linq usage in TouchCollection, don't allow null touches to be provided (matches XNA on WP7), provide our own enumerator implementation (matches XNA)

http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.touch.touchcollection.getenumerator.aspx
Merge pull request #2735 from danzel/tidy-linq

Tidy up unnecessary Linq usages
Ensure struct enumerator on TouchCollection
Merge pull request #2737 from danzel/touchcollection-enumerator

Ensure struct enumerator on TouchCollection
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