Segment for Remaining Battery Capacity

- Added a 'battery' segment function to powerline.segments.common.
  - Included tests for output of 'format', 'steps', and 'gamify' parameters.
  - Included an implementation of '_get_capacity()' for at least one linux.  However, I don't have a Mac or familiarity with other *nixes, so many other systems will throw a 'NotImplementedError'.
  - Included colors for tmux.

So, current patch is missing implementations for other operating systems and colors for using the segment in more contexts.

Ref: #504