Configurable database

[nahuelhds]

Now the database connection is handled with the db property directly and using URL database connections. If no db prop is defined, the value sqlite:///diffengine.db is used in order to maintain compatibility.

In order for this to work with PostgreSQL I've added psycopg2-binary==2.8.5 package. So, basically, with this feature the project has "native" support for Sqlite and PostgreSql. If any other engine is desired, then the proper package needs to be installed (MySQL or any other) and of course the proper database url string to be set as indicated in the peewee docs

The postgre configuration requires using ?autorollback=true at the end of the database url, in order to avoiding unwanted errors These ones were because of the emailed column that my schema was missing after the changes in #77.

I've updated the chromedriver in Travis CI in the PR #84 and I've cherry picked those commits for the CI to pass here too.

Non related

• I've added the config for time.sleep (closes Make time.sleep configurable and default to zero #76)
• I've create a suite for the feed tests and I've independizate them as well for easier testing

[nahuelhds]

@edsu this one is ready to be merged too!