WIP: use Pango and cairo through cffi.

Use cairocffi
Ditch pycairo, PyGTK and PyGobject-introspection.
More WIP: cairocffi and pango cffi.
Use pixbuf through cffi.
All tests passsing with cffi!!
Update setup.py and tox.ini
Convert pixbuf images to cairo with Gdk if available.
Update for cairocffi changes.
cairocffi accepts byte strings for set_mime_data() now.
Whitespace fixes.
Keep currentColor as a computed value, resolve in used value

According to the css3-color errata:
http://www.w3.org/Style/2011/REC-css3-color-20110607-errata.html
Don't strip leading non-breaking spaces
Have a better error message when CairoSVG uses pycairo instead of craicffi.
cairocffi is released, remove the tarball from tox.ini
Fix horizontal negative margins for floats
Import stuff early, do not try to run --help without importing.
Merge branch 'cffi'

Conflicts:
	weasyprint/text.py
Don’t bother making CairoSVG optional.
Fix the GDK-PixBuf loader for images with an alpha channel.
Make GDK-PixBuf optional.
Unicode strings are fine with CFFI 0.5