Request: Add JWT Support

[Cajunvoodoo]

It would be awesome to have JWT support in the same way we have UUID support. This shouldn't be too hard to add, since it appears that the UUID implementation isn't too complex. Please let me know if I'm mistaken here! If I have time, I might look into writing this myself.

What would be required beyond adding a new field to PGType and writing the raw UnsafeExpressions?

[echatav]

You might also need to add instances for encoding and decoding and inlining between the Haskell type and corresponding Postgres type for JWT.