Add other cryptographic funtions from pgcrypto

see http://www.postgresql.org/docs/9.3/static/pgcrypto.html
I have an error when doing that.
```python
  File "/Users/k/code/python/django-pgcrypto-fields/tests/models.py", line 3, in <module>
    from pgcrypto_fields import functions
  File "/Users/k/code/python/django-pgcrypto-fields/pgcrypto_fields/functions.py", line 3, in <module>
    from pgcrypto_fields import aggregates
  File "/Users/k/code/python/django-pgcrypto-fields/pgcrypto_fields/aggregates.py", line 3, in <module>
    from pgcrypto_fields.sql import aggregates
  File "/Users/k/code/python/django-pgcrypto-fields/pgcrypto_fields/sql/aggregates.py", line 3, in <module>
    from pgcrypto_fields import functions
ImportError: cannot import name 'functions'
```