Closes #1636.  When plotting a hexbin on a log scale, the "trick" to reuse the polygons can't work, so revert to the old method where each polygon is independent.
PEP8 fixes.
Seed the random number generator in the hexbin_log test
Merge pull request #1636 from mdboom/hexbin-log

hexbin log scale is broken in matplotlib 1.2.0