Fix #1615 -- offset is empty.
added test of 'bottom' kwarg in `hist`
fix of issue #1763

The `bottom` that needs to be passed to bar or barh got lost kwarg got
lost in the histogram refactoring.
Aternative to #1777.  Properly closes the paths returned by clip_to_rect and adds a new clip_to_bbox method to Path that returns a Path object.  Adds a test that tests clipping compound paths and multiple subsegments.
Use `np.empty` for a small speed improvement.
Document that clip_to_bbox requires closed paths.
Merge pull request #1778 from mdboom/clip_path_to_rect

Fix clip_path_to_rect, add convenience method on Path object for it
Merge pull request #1615 from mdboom/offset-empty

Offset is empty with usetex when offset is equal to 1
Merge pull request #1775 from tacaswell/issue_1763

Issue #1763
Fix regression tests.
Attempt to fix 2to3 on Travis
Only skip multiprocessing inside of Travis
Merge pull request #1784 from mdboom/travis/permission_error

Attempt to fix Travis "permission denied" error for Python 3
Re-write stacked step histogram
Fix bug in stacked bar histogram. Also, calling hist with `log=True` now
behaves correctly in all cases.
Update a few test baseline images
PEP8 fixes