Simplify code

Need to split up fields to:
  - DigestField
  - HMACField
  - PGPPublicKeyField
  - PGPSymmetricKeyField
:+1: 
```bash
  File "/Users/k/code/github.com/incuna/django-pgcrypto-fields/pgcrypto_fields/proxy.py", line 51, in __set__
    setattr(obj, self.field.name, value)
RuntimeError: maximum recursion depth exceeded
```