[xbuild] Implement the GetAssemblyIdentity task

Hi I'm lambdalice and from Nemerle Forum.

We'd had a problem with XBuild, the one of them is that ```GetAssemblyIdentity``` is not implemented.

I've implemented the GetAssemblyIdentity.cs but I don't know how to register it.

It says:

```
: error : Error initializing task GetAssemblyIdentity: Not registered task GetAssemblyIdentity.
```

Please manage it successfully... :(

Please see also our discussion:
https://groups.google.com/forum/#!topic/nemerle-en/xNrjBBqtoYE