Added async/await to Dapper.Rainbow - .Net 4.5 Hooray \o/

I kept my hands off Sam's original Code as much as possible, added the original project/files as links into the new NET45 project and only made the two major classes partial and added the .Net 4.5 Async other half in the new project's file (basically what Dapper NET45 does to Dapper).

I also moved the original Dapper.Rainbow.nuspec one level up to be side-by-side to the rest of the .nuspec files and updated it a bit (hope that's fine) but primarily to also include the net45 file(s).

In any case, Dapper.Rainbow could need some general loving - I tried to adhere Sam's original code style in the project but documentation is missing throughout and no tests doesn't feel good. Moreover the new *Async() methods bring some code redundancies which I would strip away into a common place, but oh well.

All that wasn't necessary to add plain async/await functionality but if it's ok, I might go ahead and send in future pull requests for it.

But for now - async Dapper.Rainbow is here :)