Add ExecuteReaderAsync overloads that return Task<DbDataReader>

[jnm2]

Closes #1140

In order to avoid a binary breaking change, new overloads must be added rather than changing the return type of the existing methods.

This causes a potential source-breaking change because the new DbConnection overloads will take precedence over the existing IDbConnection overloads when users recompile their projects:

// CS0029: Cannot implicitly convert type
// 'System.Threading.Tasks.Task<System.Data.Common.DbDataReader>'
// to 'System.Threading.Tasks.Task<System.Data.IDataReader>'

Task<IDbDataReader> readerTask = dbConnection.ExecuteReaderAsync(...);

// Or

Foo(dbConnection.ExecuteReaderAsync(...));

void Foo(Task<IDbDataReader> readerTask)
{
    // ...
}

/cc @mgravell

[mgravell]

looks good, thanks; merging