Support custom plugin locations and arbitrary plugin modules

[csams]

Allow users to write their own plugins without needing to download the project repo. They can install did and use the config file to point to their custom plugins. This also makes it easier to share plugins outside of the project itself.

• Refactor the plugin loading logic to use importlib and pkgutil so it can load plugins from an egg (already on the PYTHONPATH) as well as arbitrary places on the file system.
• Add a custom_plugins option to the general section of the config. Users can add a space or comma separated list of directories where custom plugins reside, and they'll automatically get loaded if the directories are valid python2 packages (include __init__.py files).
• Refactor the plugin system so StatsGroup subclasses register themselves. This removes the need to scan modules individually and allows plugins to be defined at any level in a package hierarchy.

This PR introduces no new dependencies, but it does use importlib, which only comes installed with python 2.7 and beyond. It's available in pypi for python 2.6. The new loader code is python2/3 compatible.

Example config:

[general]
email = Joe User <joe@example.com>
custom_plugins = ~/.did/custom_plugins

[thing]
type = thing

Example file (thing.py) in ~/did/custom_plugins:

from did.stats import EmptyStatsGroup


class Thing(EmptyStatsGroup):
    """ This is a Thing! """
    pass

Example help:

└> did -h
usage: did [this|last] [week|month|quarter|year] [options]

optional arguments:
  -h, --help            show this help message and exit

Select:
  --email EMAILS        User email address(es)
  --since SINCE         Start date in the YYYY-MM-DD format
  --until UNTIL         End date in the YYYY-MM-DD format

This is a Thing!:
  --thing               All above

Format:
  --format FORMAT       Output style, possible values: text (default) or wiki
  --width WIDTH         Maximum width of the report output (default: 79)
  --brief               Show brief summary only, do not list individual items
  --verbose             Include more details (like modified git directories)

Utils:
  --config FILE         Use alternate configuration file (default: 'config')
  --total               Append total stats after listing individual users
  --merge               Merge stats of all users into a single report
  --debug               Turn on debugging output, do not catch exceptions
  --test                Run a simple smoke test against the github server

[coveralls]

Coverage increased (+0.07%) to 81.972% when pulling b08a934 on csams:refactor_plugin_loading into 2b73b75 on psss:master.

[csams]

@psss Would you rather this PR go against devel?

[psss]

Thanks for the enhancement. This looks good. I just need a bit more time to review the changes. Hopefully in the coming days. No need to update this against the devel branch.