#12: bumped css-selector requirements

Add function to check if HTML snippets exist

Merge pull request #10 from krzysztof-pyrzyk-ah/bugifx/fatal-error-wh…

…en-no-result-cache-provided

Fix fatal error when no cache provided in constructor

Merge pull request #9 from JanDC/ignore-improper-selectors

If no expression can be build, the selector is invalid

Expanded rules cache to included negatives / provide html cache ident…

…ifier to prevent collisions across pages

Cache rules further by storing the combination of css/rules

Cache expressions

Merge pull request #6 from tvlooy/master

Revert

Merge pull request #5 from tvlooy/empty_html

avoid warning that html is empty

Merge pull request #2 from tvlooy/implement_abstract_method

Implement Twig_ExtensionInterface::getName