Free outname
Support liq_attr_create_with_allocator(NULL,NULL)
Re-add zlib as a dependency

Some builds of libpng don't have zlib baked in
Prevent conflict with libpng16 by renaming png_image with rwpng_png_image
Fixed reference to png_image
Added include required for debug build
Linking needs more cowbell and libm
Documented color difference formula
Correct palette index guesses when smaller palette is picked

Fixes #39.
Documentation
Erase palette when freeing it

To make use-after-free more obvious