Affine

------------------
- Breaking change: precision used in properties like ``is_conformal`` is no
longer a global module attribute, but an Affine class or instance attribute
(19, 20).
- Breaking change: ``is_degenerate`` property is now exact and not subject to
a level of precision (23).
- Breaking change: we have reversed our sense of rotation, a positive angle
now rotates a point counter-clockwise about the pivot point (25).
- Bug fix: a bug in matrix-vector multiplication had been reversing the
direction of rotation and is now fixed (25).

------------------
- is_degenerate predicate is precise, not approximate (22).

------------------
- Enable pickling of Affine objects (14).
- Sort out the mixed up shearing parameters (12).

------------------
- Add loadsw/dumpsw world file utilities (6).
- Travis-CI and Coveralls config and web hooks added (10).

------------------
- set_epsilon() now actually sets module EPSILON (4).
- add AUTHORS.txt.

----------------
- Code ported from Casey Duncan's Planar package.
- from_gdal() class method added.