Solution: BigchainDB by default writes logs in the same directory it is
run. The `looponfail` feature provided by pytest waits for changes in
the current directory, so it is continuously triggered. This patch tells
pytest to only watch the `bigchaindb` and `tests` directories.
* Remove testing for rethinkdb, mongodb, and Py3.5
* Add first tests
* Add validation
* Add command to start the ABCI Server
* Reuse existing MongoDB Connection class
* Use DuplicateTransaction
* Test only tendermint
* Update travis scripts
* Fix pep8 errors
* Update Makefile
