updated docstrings
update gitignore
FIX: bug fixes, improved test coverage

sort_by_value() and PQDict.create() were broken (oops!).
consume() wasn’t working properly either. None of these late additions
had been unit tested, of course. *facepalm*

Also changed sort_by_value to return a generator rather than a list.
PQDict.fromkeys keyword sort_by was changed to rank_by.
add ga to docs
add travis.yml
Merge branch 'master' of https://github.com/nvictus/priority-queue-dictionary