Do not require Django configured for installing django-pgcrypto

I've installed ``django-pgcrypto==1.1.1`` without problems, but was impressed with further changes in 1.2.0 (especially Django 1.7 and Python 3 support) that tried to reinstall it from source, but cannot complete installation cause of ``DJANGO_SETTINGS_MODULE`` env var not supplied on time of installation (which is expected behavior):

    (venv)$ pip install -e git+https://github.com/dcwatson/django-pgcrypto.git@8d3a5929210a3e9fd81d9be49c8cff2fca2119f4#egg=django-pgcrypto
    Obtaining django-pgcrypto from git+https://github.com/dcwatson/django-pgcrypto.git@8d3a5929210a3e9fd81d9be49c8cff2fca2119f4#egg=django-pgcrypto
      Cloning https://github.com/dcwatson/django-pgcrypto.git (to 8d3a5929210a3e9fd81d9be49c8cff2fca2119f4) to ./venv/src/django-pgcrypto
      Could not find a tag or branch '8d3a5929210a3e9fd81d9be49c8cff2fca2119f4', assuming commit.
      Running setup.py (path:/home/playpauseandstop/Projects/getgoing/airborne/venv/src/django-pgcrypto/setup.py) egg_info for package django-pgcrypto
        Traceback (most recent call last):
          File "<string>", line 17, in <module>
          File "/home/playpauseandstop/Projects/getgoing/airborne/venv/src/django-pgcrypto/setup.py", line 2, in <module>
            import pgcrypto
          File "pgcrypto/__init__.py", line 18, in <module>
            from .fields import *
          File "pgcrypto/fields.py", line 5, in <module>
            from django.db import models
          File "/home/playpauseandstop/Projects/getgoing/airborne/venv/lib/python2.7/site-packages/django/db/__init__.py", line 11, in <module>
            if settings.DATABASES and DEFAULT_DB_ALIAS not in settings.DATABASES:
          File "/home/playpauseandstop/Projects/getgoing/airborne/venv/lib/python2.7/site-packages/django/conf/__init__.py", line 53, in __getattr__
            self._setup(name)
          File "/home/playpauseandstop/Projects/getgoing/airborne/venv/lib/python2.7/site-packages/django/conf/__init__.py", line 46, in _setup
            % (desc, ENVIRONMENT_VARIABLE))
        django.core.exceptions.ImproperlyConfigured: Requested setting DATABASES, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.
        Complete output from command python setup.py egg_info:
        Traceback (most recent call last):

      File "<string>", line 17, in <module>

      File "/home/playpauseandstop/Projects/getgoing/airborne/venv/src/django-pgcrypto/setup.py", line 2, in <module>

        import pgcrypto

      File "pgcrypto/__init__.py", line 18, in <module>

        from .fields import *

      File "pgcrypto/fields.py", line 5, in <module>

        from django.db import models

      File "/home/playpauseandstop/Projects/getgoing/airborne/venv/lib/python2.7/site-packages/django/db/__init__.py", line 11, in <module>

        if settings.DATABASES and DEFAULT_DB_ALIAS not in settings.DATABASES:

      File "/home/playpauseandstop/Projects/getgoing/airborne/venv/lib/python2.7/site-packages/django/conf/__init__.py", line 53, in __getattr__

        self._setup(name)

      File "/home/playpauseandstop/Projects/getgoing/airborne/venv/lib/python2.7/site-packages/django/conf/__init__.py", line 46, in _setup

        % (desc, ENVIRONMENT_VARIABLE))

    django.core.exceptions.ImproperlyConfigured: Requested setting DATABASES, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

    ----------------------------------------
    Cleaning up...
    Command python setup.py egg_info failed with error code 1 in /home/playpauseandstop/Projects/getgoing/airborne/venv/src/django-pgcrypto
    Storing debug log for failure in /home/playpauseandstop/.pip/pip.log

So I created this pull request, where instead of importing ``pgcrypto`` in setup script I reading version info from module content and making version number from it.