Add box2d type

[D4no0]

I am trying to do a select st_envelope(...)::box2d and I get the error:

* (Postgrex.QueryError) type `box2d` can not be handled by the types module Test.PostgresTypes

[bryanjos]

We don't have any support for box2d yet. I just did a quick search on trying to figure out how what the specs are for it but haven't found anything yet. But yes, support should be added