Support multiple databases

[copelco]

Update django-pgviews to support multiple databases (per #9):

• Adds multiple database instructions to README.
• A --database option may now be provided to the management commands to specify a non-default database.
• By default, django-pgviews continues to use the default database and no changes are required for this use case.
• Includes multi-database related unit tests in tests/test_project/test_project/multidbtest.

[mikicz]

This is pretty much perfect from what I can see!

I ran the tests locally and it all passed (no idea why CircleCI didn't run tests), I also tried using it in my company's project and it all works great there as well, so merging this and will release it.

[si14]

Thanks a lot @mikicz and @copelco, this is great!