Updated Sqerl for latest epgsql/epgsql

The PR You've ALL been waiting for.

* Updated `rebar.config` to use [epgsql/epgsql](https://github.com/epgsql/epgsql)
* Updated error code reporting to use epgsql's error type
* epgsql doesn't *yet* support query timeouts, so using a suggestion from their mailing list, call `set statement_timeout=$default` before every query.

*FUTURE WORK*
Now that each query sets a timeout to the default value, we can add versions that let you specify the timeout you want for a specific query. The goal of this PR was parity with the [wg/epgsql](https://github.com/wg/epgsql) functionality, which has been achieved. (except on connection times, but you can't have it all, Veruca)



You mention `wg/epgsql` in the comment, but we're using `epgsql/epgsql`, correct?

Is the comment just out of date?