Tests for #544
Sane quantize defaults for RGBA->P, fixes #544
Changes.rst updated
Tests for #510
Transparency conversion for L, RGP and P images, fixes #510
Updated Changes
Better tests, fixes for leaking RGB->P transparency
Merge from master
warn category defaults to UserWarning
Trapping warnings
Merge pull request #574 from wiredfool/convert_trns

Indexed Transparency handled for conversions between L, RGB, and P modes