MDoc and monodoc.dll changes to support iOS 64Bit API

These changes are to support the upcoming Xamarin.iOS API. It gives mdoc and monodoc the ability to update a set of ecmaxml docs from two separate assemblies. There are two major features:

- --dropns - this flag tells mdoc that a namespace has been dropped (ie. MonoTouch) in the new assembly, and as such to take this into consideration when matching member and type signatures.
- -ntypes - there are a set of new types that are being used in Xamarin.iOS (you can see them listed [here](https://github.com/joelmartinez/mono/compare/docs-64bit-squash?expand=1#diff-e371a9a9ab867a3963916a0568193b14R28) ), and so this flag tells mdoc to try to match members using the original types when it encounters one of these in the new assembly.
- -preserve - tells mdoc to avoid removing members that are not present in the current assembly ... to avoid removing members that have been dramatically altered in the other assembly.

Running with these flags will add the alternate details to the same member node in the XML, thus preserving existing documentation, and making it so that documentation writers do not have to maintain to separate but similar sets of documentation.