add support for transparant png
submodule updated
Remove unused files
auto merge of #1291 : sammykim/servo/unused-file, r=jdm
auto merge of #1288 : ryanhc/servo/transparent_png, r=larsbergstrom

Modified to use libpng to read .png files. We use stb_image to read all other image files.
This requires updated submodule: https://github.com/mozilla-servo/rust-png/pull/13

This patch is for:
https://github.com/mozilla/servo/issues/1279
Disable parallel builds at top level.

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650
auto merge of #1292 : metajack/servo/not-parallel, r=larsbergstrom

Sub-builds are still parallelized. This is a workaround for rustpkg
issues. See https://github.com/mozilla/rust/issues/9650