Bidirectional Converters and Quirks

* Converters can go both directions, see https://github.com/aaberg/sql2o/issues/65
* Quirks is now an interface rather than an enum, see https://github.com/aaberg/sql2o/issues/62

Before merging, I want to test on a DB2 database because the `Query#addParameter(String, Date)` no longer exists. @aaberg do you have a DB2 db available to you?

**Note:** at some point we may want to move the quirks implementations to respective extension packages