A new user-defined operator try
Merge branch 'upstream_master' into new-user-defined-operator-try

Conflicts:
	ICSharpCode.NRefactory.CSharp/Resolver/CSharpConversions.cs
	ICSharpCode.NRefactory.Tests/CSharp/Resolver/ExplicitConversionsTest.cs
Look at implicit and explicit user-defined conversion operators at the same time when performing an explicit conversion
Return ambiguous conversions when no most specific source and/or destination could be found.