support for POINT class

Since node-mysql supports [SELECTing and casting the POINT class](https://github.com/felixge/node-mysql/blob/0c99f5639f4c28ce10fabbac9523b58b61f508d1/lib/protocol/Parser.js#L311), it would make sense to also support escaping `POINT`s during INSERTs and UPDATEs. Currently, if you SELECT a field with a point, it'll return a plain object with `x` and `y` attributes. This PR will allow it to also INSERT and UPDATE an object with `x` and `y` attributes.

I have no idea if this was the correct way to append a new type in `SqlString`, but it works fine for me. The POINT function is defined [here](https://dev.mysql.com/doc/refman/5.0/en/data-types.html).