* Problem: Validator set not tracked by BigchainDB
Solution: BigchainDB depends on tendermint's RPC API to get the validator set
which is not avaiable during replay so the validators set should be tracked
inside BigchainDB
* Problem: Unclear code and documentation
Solution: Fix decode_validator and docs strings
* Problem: Doc strings missing
Solution: Add doc string for store_validato_set
* Problem: voting code is not used anymore
Solution: remove all voting related code
* Problem: Some voting functionality is still present.
Solution: Remove it. Update some of the related tests.
* Problem: some skipped tests are now running
Solution: remove pytest mark to not run them
* Problem: fastquery is not related to votes
Solution: remove it in another PR
* Problem: RethinkDB, change feed, old mongo, admin interface are not supported any longer.
Solution: Remove unsupported functionality. Bring the MongoDB backend implementation completely to the localmongodb package. Fix the test setup.
* Problem: Nothing depends on multipipes any longer.
Solution: Remove multipipes from setup.py.
* Problem: The how-to-run-tests doc uses --database-backend.
Solution: Do not include the --database-backend option into the documented pytest usage.
* Problem: The backends docs are outdated.
Solution: Document MongoDB as the default and only backend for BigchainDB.
* Problem: The inputs fixtures uses old blocks API.
Solution: Change the inputs fixtures to use the new blocks API.
* Problem: rethinkdb package is not used anymore.
Solution: Remove the rethinkdb dependency from setup.py.
* Problem: The abci-marked tests use outdated Mongo conn.
Solution: Replace MongoDBConnection with LocalMongoDBConnection for them.
* Problem: Amount error is not tested on the HTTP level.
Solution: A failed web test to reproduce the problem.
* Problem: Invalid transaction posted to Tendermint
Solution: Pass the exception to HTTP POST handler function
* Problem: DoubleSpend and CriticalDoubleSpend not differentiated
Solution: Handle these exceptions differently in `get_spent`
* Problem: No test for checking exception DoubleSpend and CriticalDoubleSpend
Solution: Add necessary tests
* Problem: find doesn't raise IndexError
Solution: Remove exception handling for IndexError
* 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
