* Add SSL support for MongoDB connections
* Tests for TLS connectivity and other fixes
* Add test for ssl parameters
* Add test for AuthenticationError
* Cleanup branch
* Split env vars as per @r-marques suggestion
* Remove SSL_ENABLED and use BIGCHAINDB_DATABASE_SSL instead
* Changes as per comments from @r-marques
* Remove redundant tests
* Test for ConfigurationError
* 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