Cleanup: Use "cv2.cv.CV_FILLED" instead of "-1" magic number
Refactor: Don't do any pyramid level changing inside `_match_template`
Refactor
_find_match is a generator
WIP: API: Add find_all

TODO:

* Documentation
* Tests for annotation
* Reduce duplication
fixup! WIP: API: Add find_all

(Fixes merge error)
Rename `find_all` to `match_all` (and `find_iter` to `match_iter`)

For consistency with `match`.
move `match_all` tests from stbt/__init__.py to tests/test-match.py
stbt.match_all returns an iterator; remove stbt.match_iter

Python seems to be going in the direction of everything being lazy if
possible (Python 3's `range` behaves like Python 2's `xrange`, etc).
You can use `list(match_all)` if you need to force the evaluation.
stbt.match_all docstring
fixup! stbt.match_all returns an iterator; remove stbt.match_iter