Django-pgcrypto-fields

* Rename package from `pgcrypto_fields` to `pgcrypto`.

* Make `get_placeholder` accepts a new argument `compiler`
* Fix buggy import to `Aggregate`

**Note: these changes have been done for django > 1.8.0.**

* Remove `MaxLengthValidator` from email fields.

* Avoid setting `max_length` on PGP fields.

* Allow/check `NULL` values for:
`TextDigestField`;
`TextHMACField`;
`EmailPGPPublicKeyField`;
`IntegerPGPPublicKeyField`;
`TextPGPPublicKeyField`;
`EmailPGPSymmetricKeyField`.
`IntegerPGPSymmetricKeyField`.
`TextPGPSymmetricKeyField`.

* Fix `cast`ing bug when sending negative values to integer fields.