Adding TextPlainFormatter to always handle returning strings as text\plain format.

Conflicts:
	src/Microsoft.AspNet.Mvc.Common/Encodings.cs
	src/Microsoft.AspNet.Mvc.Core/Formatters/JsonOutputFormatter.cs
	src/Microsoft.AspNet.Mvc/MvcOptionsSetup.cs
	test/Microsoft.AspNet.Mvc.Core.Test/ActionResults/ObjectContentResultTests.cs
	test/Microsoft.AspNet.Mvc.Test/MvcOptionSetupTest.cs