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

I kept my hands off Sam's original Code as much as possible, only made
the two major classes partial and added the .Net 4.5 Async other half in
the new project (basically what Dapper NET45 does to Dapper).
Deleted unnecessary AssemblyInfo.cs

.. the original one is linked in.
Merge pull request #162 from jbattermann/master

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