Improved tests

Improved tests since they wouldn’t fail if connection was successful
and db.conn returned None. Tests include now table creation, so things
going wrong can be easily spotted. Added table cleanup also; since we
do not have the connection object globally availble, we could not write
a tearDown() method that could handle table cleanup.
Added db argument to RethinkDB() constructor

Added the possibility to choose to which database to connect when
initializing the RethinkDB object.