Fixed blurry icon issue

Bitcoin.png in src/qt/res/icons/bitcoin.png was wrong canvas size
(should have been 256x256-was 16x16).  Also, updated the
build-macosx.md documentation in /doc/build-macosx.md file to include
the headerpad linker modifier so that dynamic libraries would properly
add to the APP bundle (latest build of miniupnpc broke the build).