Added method to use AnsiString as Default

Dapper's default string handling is Unicode string. This PR is to add a method to be able to change the default string handling to Ansi String.

Applications that want to use the Ansi String default will need to make the following method call before any DB calls with Dapper to change the Type Mapping.

Dapper.SqlMapper.AddTypeMap(typeof(string), DbType.AnsiString);