Support for SmallInteger
No, I'll whip up an implementation, seems useful.
 Beautiful, thank you!

Doesn't appear to be a cross database compatible field for representing `smallint`.

[Postgres](http://www.postgresql.org/docs/9.1/static/datatype-numeric.html), [MySQL](https://dev.mysql.com/doc/refman/5.5/en/integer-types.html) and [SQLite](https://www.sqlite.org/datatype3.html) seem to handle `smallint` in the same way, with the exception of SQLite casting all `smallint` to `int`.

Any objections for adding this to the core? Happy to send a PR if needed
