Expose DataRowReader FieldDescriptions
Prepare returns *PreparedStatement
Encoders for ints and floats now convert compatible values

* Any int that fits in the destination int type will be accepted
* Any float that fits in the destination float type will be accepted
Fix CommandTag RowsAffected for INSERT
Fix example url_shortener
Add initial database/sql support
DRY pgx/stdlib tests
Make Conn Close idempotent

* die (which is called by Close) now closes underlying connection
Do not share database connections between tests
Allow running tests in parallel
Inform database/sql when connections die