add function 'zopfli_png_recompress'

recompress IDAT chunk in PNG Image with zopfli
move png utility from zopfli.c to png.c
max_size is not used
add description for zopfli_png_recompress
forgot to add png.c
Updated zopfli_png_recompress
Added png.h
Added tests/zopfli_png_recompress.phpt