Add isempty, lower_inc, upper_inc, lower_inf, and upper_inf range lookups

[Peter J. Farrell]

Add support for following functions for Postgres range fields that are currently not supported in Django:

• lower_inc
• upper_inc
• lower_inf
• upper_inf

​https://www.postgresql.org/docs/9.3/static/functions-range.html

Example usage:

MyObject.objects.filter(date_range__upper_inf=True)  # for infinity
MyObject.objects.filter(date_range__upper_inf=False)  # for not infinity

Example code: ​https://gist.github.com/peterfarrell/5c7b8d80319563813caba6320b568829

These can all be implemented as new ORM lookups for the Postgres range type fields so the Django ORM can gain parity with functionality available in Postgres range fields.

[Peter J. Farrell]

I am volunteering to make a PR to add support for these 5 functions. I will start work on a PR (including tests and documentation) if this ticket is accepted.

I have two working custom lookups (lower_inf and upper_inf) already built (see the gist linked in the main ticket). They need to add casting for different range types to improve compatibility however the concept is sound.

[Peter J. Farrell]

The isempty postgres function is already implemented and documented. However, lower and upper functions are implemented by as startswith and endswith which is a bit confusing unless you look at the code. So I am planning on improving the documentation to indicated which postgres function those lookups map to.