Bump version to 2.1-dev for development
pngcrush

Running all textures through pngcrush-1.7.70 from http://pmt.sourceforge.net/pngcrush/
pngcrush -rem alla -rem text -d /tmp/textures/items/ textures/items/*
pngcrush -rem alla -rem text -d /tmp/textures/blocks/ textures/blocks/*
cp /tmp/textures/items/ textures/items/
cp /tmp/textures/blocks/* textures/blocks/

Closes GH-8