Docker files updates

[sbellem]

This PR updates the docker-compose file so that:

1. MongoDB can be used as a backend when running BigchainDB server.
2. The websocket events API can be used.

Note for @ttmc this will most likely require minor changes to docs. Will add you as a reviewer once the docs have been updated.

• [x] update docs

[codecov-io]

Codecov Report

Merging #286 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #286   +/-   ##
=======================================
  Coverage   99.15%   99.15%           
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  Files           9        9           
  Lines         237      237           
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  Hits          235      235           
  Misses          2        2

[ssadler]

I like the simpler docker-compose file, with db and bdb-server. I wonder if bdb isn't a little too general, though, since it's actually the driver that it's running.

[sbellem]

I wonder if bdb isn't a little too general, though, since it's actually the driver that it's running.

This is from a client point of view. We have been using bdb in the client's docs to refer to an instance of the client for example.