Update usage.rst
get css backgrounds and fonts relative to the css file path
rename do_under_line to _do_under_line
Fix a sneaky bug in the css cache

When you call pisaParser the first time it's okay but on subsequent calls the pdf is pseudo-randomly corrupted.
    
1. We need node.tagName in getCSSAttrCacheKey.
    
2. CSSAttrCache in pisaParser() should be global else initialization has no effect.
Merge pull request #110 from abravetti/master

Fix a sneaky bug in the css cache
Merge pull request #111 from sbisinger/master

Update the documentation to include <pdf:pagecount />
Merge pull request #109 from mthornhill/master

Fix name of _do_under_line method
Merge pull request #104 from pretaweb/master

get css backgrounds and fonts relative to the css file path
Added sbisinger, abravetti and mthornhill to AUTHORS.

Thanks a lot for your contribution!
Merge pull request #103 from leafweyo/patch-1

Update usage.rst