* Fix typos pointed out in review by @ttmc
* Reword description of an input in the transaction schema
* Re-add removed comment in transaction model
* Fix small typos in some comments in the transaction model
* Add trailling commas to a multiline dict in the transaction model tests
* Fix small things with server docs changes
* Add description of public keys' association with conditions in transaction concept docs
* Reword description of Transaction.create's and args
* Rename output_uri to output_condition_uri
* Fix hardcoded class name vs. self.__class__
* Rename instances of 'out' with 'output'
* Rename instances of and with to avoid name clash with built-in
* Remove unnecessary renaming of cryptoconditions.Fulfillment import in transaction model
* Remove instances of in transaction model
* Remove usages of fulfillment in cases where input makes more sense
* Reword docstrings for init methods in transaction models
* Rename usages of condition where output is now a better fit
* Add descriptions to TransactionLink's txid and idx in schema
* Minor correction to output idx description in transaction yaml
* Remove unnecessary import of rethinkdb in a test
* Move test_run_query_util as a rethinkdb-specific test
* Move rethinkdb-specific command tests to own file
* Add __init__.py to new test folders
* Move command tests to their own test module
* Move fixtures out of command tests into separate conftest for commands
* Fix some small flake8 issues with rethinkdb's test_run_query_util
* Add pytest ignore hook to filter out test dirs that are not for the requested backend
* Move backend-specific tests in tests/db/ to tests/backend/
* Add README.md to tests/ to describe test structure and pytest customizations
* Add package-level docstrings to backends
* Add README.md explaining the backend architecture
* Small improvements to docstrings in backend modules
* Restructure the backend automodule docs
* Add more docstrings to backend connections
* Add init to base backend Connection class to document expected interface
* Reword the backend/README.md docs
