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