Add documentation, release process, and improve testing

[lgo]

Lumping this all together as these are all improvements that just come with better support for the adapter.

• [ ] Create a proper release process
  • [ ] Releasing to rubygems. (Look into using TravisCI to automatically publish tags)
  • [ ] Changelog
  • [ ] Tagging
  • [ ] Make sure to support Activerecord 4.x (our 1.x) and 5.x (our 2.x) :clap:. This could either be using two different branches, or supporting them from the same branch (sort of like Py2 vs Py3 support).
• [ ] Add documentation (and changelog)
• [ ] Improve testing
  • [ ] Possibly pull in existing ActiveRecord PostgreSQL tests. Again, we'll have to do this separately for the different ActiveRecord versions likely.
• [ ] Add rake tasks to manage the database
• [ ] Add custom CockroachDB capabilities
  • [ ] Don't use PostgreSQL types, but use cockroach types. We'll then alias PostgreSQL ones to their respective CockroachDB one

[lgo]

I'm currently working on getting the tests working and there are two possible approaches:

1. Duplicating the PostgreSQL tests in the main ActiveRecord repo to run tests from there (on this fork)
2. Getting those same tests to work from this repository instead, so we don't need a modified ActiveRecord. (Experimenting on this branch)

Approach 2 is a lot more ideal as it will be easier to have AR4 and AR5 tests and keep everything in this repo. We'll just need a way to leverage test helpers from outside the ActiveRecord gem.