Fixed the use of a unimported Exception class in the code copied
from the PrinceXML project.

This bug was detected by SensioLabs Insight (rule # 22-080):

"Missing use statement should be avoided."
Removed some commented code.

This bug was detected by SensioLabs Insight (rule # 25-085):

"Commented code should not be commited"
Added some PHPDoc to remove an "unused use statement" error
Refactored some Twig templates used to generate CSS fragments.

SensioLabs Insight suggest that Twig templates should be less than
200 lines long (rule # 37 - 010). To achieve this goal, some CSS rules
have been collapsed and the typography-related @font-face rules have
been extracted into separated Twig fragments.