Added support for postgres raw encryption functions.

[stas]

This introduces a new field type for playhouse.postgres_ext to handle encrypted columns.

This should work with AES and Blowfish algorithms. The encryption is handled by the pgcrypto module.

Although this works, this is not very optimized. See #401
I opened this PR mostly because I'm interested in having encryption support in peewee.
Although this is pretty raw, I hope to get some feedback and other people review and play with this.

Thanks a lot for all the work on peewee!