Added full issue milestone model.
Include milestone information in default templates.
[QA] Minor whitespace fixes.
Clear os.environ when running editor handling tests.

The partially modified dictionary in the previous caused failures when using
git's --message option, as it sets GIT_EDITOR to ':'.
Added basic repository milestone listing.
Added simple milestone assigning for issues.
Merge branch 'feat/milestones'

Closes #40.

* feat/milestones:
  Added simple milestone assigning for issues.
  Added basic repository milestone listing.
  Include milestone information in default templates.
  Added full issue milestone model.
Fix base exception handling.

This fixes a bug introduced way back in
671b031769de79453e6c58f5d2a1d16cceb011a8, ick.
[QA] Bump copyright years.
[QA] Include patch authors in copyright notice.
0.16.0 released.