Improve .NET Integration

A while ago Amit Regmi, a student from Canada, started working on the clr module improvements branch as a university project.

During the sprint Carl Friedrich, Paul and me worked more on it and brought it to a mergeable state.

It adds a lot of new features to the clr module, which is the module that allows integration between pypy-cli (aka PyPy.NET) and the surrounding .NET environment:

• full support to generic classes;
• a new importer hook, allowing things like from System import Math and so on;
• .NET classes that implements IEnumerator are treated as Python iterators; e.g. it's is possile to iterate over them with a for loop.

This is an example of a pypy-cli session:

>>>> from System import Math
>>>> Math.Abs(-42)
42
>>>> from System.Collections.Generic import List
>>>> mylist = List[int]()
>>>> mylist.Add(42)
>>>> mylist.Add(43)
>>>> mylist.Add("foo")
Traceback (most recent call last):
  File "<console>", line 1, in <interactive>
TypeError: No overloads for Add could match
>>>> mylist[0]
42
>>>> for item in mylist: print item
42
43

This is still to be considered an alpha version; there are few known bugs and probably a lot of unknown ones :-), so don't expect it to work in every occasion. Still, it's a considerable step towards real world :-).