Added ExecuteReader method

ExecuteReader returns an IDataReader that can be used to process the
results without Dapper's mapping - for example, to fill a DataTable or
DataSet
Merge pull request #103 from JJoe2/master

Added ExecuteReader method

Note: This has a few kinks (I think it closes overly aggressively, and there's too many overloads for comfort), but I'll merge it and edit those immediately afterwards