Replace most rule classes with R objects

[daisylb]

Fixes #9.

You can read documentation for this branch here: http://storage.googleapis.com/bridgekeeper-docs/refs/heads/r-objects/index.html

[daisylb]

Some to-do items courtesy of feedback from @rixx:

• [ ] Add a way to match against the current time. Currently it's possible to do R(something=lambda _: timezone.now()), but that's not pretty.

• [ ] Add support for lookups, like __lte. Currently R assumes that lookups aren't used. Ideally, it should detect which parts of a kwarg key would be interpreted by Django as lookups, and then if that lookup is supported, use a Python implementation of the lookup with the same semantics in .check(), and throw an error otherwise.

  Ideally, this error would be thrown when the R object is created, because it's better to fail early, but I'd need to look at how lookups work in Django; I'm concerned that whether something is following a FK or a lookup might be selectively determined based on metadata about the model, which isn't available to us at the time R's constructor is called. (For instance, does Django determine the meaning of foo__lte by checking either to see if foo is of a type that supports the __lte lookup, and/or checking if it's a FK to a model with a lte field?)

• [ ] Add a page of examples to the docs, that goes into more detail than the tutorial, but is more useful for people that are example-oriented than the API reference.
• [ ] Add at least one example where an R takes multiple kwargs.

I also want to do a bunch more work on the guides in the docs, but that's mostly separate to this ticket.

[daisylb]

I accidentally closed this while renaming the main branch :S

[daisylb]

This is now merged 🎉