Created a new battlecry system.

Battlecries now work with the tag system, like deathrattles and effects.

The old system is still supported while in transition, but all new cards
will be created using the new battlecry system.

Also implemented a choose mechanic and switched all Druid minions

Implemented Antique Healbot and Anodized Robo Cub as test cases for
the new system.
Generated a JSON description for all minions

Not all minions are correctly described; most minions with battlecry
or combo effects will not work.

Eventually, we can replace the Python implementation of cards with
their description in JSON, but that day is not today.

All minions from Druid, Hunter and Mage are working, along with
a majority of minions from the other classes and neutral.
Fixed the import bug that sometimes caused the tests to fail.

Now in game_objects.py, classes from the tags packages are imported
in the function they are used, rather than globally.  It's possible there is a
better way to do this, but I don't know what that way is.
flake8
Merge pull request #14 from danielyule/master

Merge