Django-pgcrypto-fields

* Merge `.coveragerc` into `setup.cfg`
* Added `.gitignore` file
* Updated out-dated requirements (latest versions of `Flake8` and `pycodestyle`
are incompatible with each other)
* Updated `README` with better explanations of the fields
* Implemented DatePGPPublicKeyField and DateTimePGPPublicKeyField

* Added support for Django 2.x+
* Updated requirements for testing
* Updated travis config with Python 3.6 and additional environments

Thanks to peterfarrell:
* Add support for `DatePGPSymmetricKeyField` and `DateTimePGPSymmetricKeyField`
including support for serializing / deserializing django form fields.
* Add support for auto decryption of symmetric key and public key fields via
the PGPManager (and support for disabling it in the Django Admin via the PGPAdmin)

* Remove compatibility for `Django 1.7`.
* Add compatibility for `Django 1.10`.
* Add `Django 1.9` to the travis matrix.

* Exclude tests app from distributed package.