Allow empty values in EncryptedDateField

[petebrowne]

Example:

class SomeDjangoModel(mode.Model):
    datetime = pgcrypto.EncryptedDateTimeField()

instance = SomeDjangoModel()
# ValidationError: [u"'' value has an invalid date format. It must be in YYYY-MM-DD format."]

Need to check if value is None or empty before parsing.

Also, can you release these changes to PyPI?

[dcwatson]

Pushed out version 1.1.1.

[petebrowne]

Thanks!