Use separate vars to track whether lookbehind is being used and the length of the lookbehind. Fixes #109.
Merge branch 'gh-pages' into bug-109-zero-length-lookbehind. Manually adjusted whitespace collision in prism.js.

Conflicts:
	prism.js
Merge pull request #110 from Mr0grog/bug-109-zero-length-lookbehind

Prevent zero-length lookbehinds from throwing off future matches