Testable Dapper

Hi, I really love Dapper and have great use for it. Removing that plumbing pain whilst also providing hyper performance in my very high volume app. 

My contribution focuses around the testability aspects of Dapper, introducing some simple interfaces to mock against and creating a wrapper implementation over the static extension methods. 

Whilst there is a Dapper Wrapper project out there this fell short for me by not wrapping the QueryMultiple member. This is due to the dependency on GridReader which, in turn, required sliding a little interface behind so we could mock out within our repositories.

The only clunk I can see to speak of is the creation of 
```c#
public interface IDynamicParameters 
```
as i guess its at odds with the nested 
```c#
    static partial class SqlMapper
    {
        public partial interface IDynamicParameters
...
```
I would personally drop the 
```c#
void AddParameters(IDbCommand command, Identity identity);
```
member into the new non nested interface but didn't as this could be a breaking change?

Anyhow, hope you like my modest changes and merge up.
(Quite excited as its my first social code contribution ever!)

I await your feedback...
Thanks
J