Fix invalid selector exception with empty class values

After removing :before and :after pseudo selectors, a class name may be empty, causing an invalid selector string when joined. Remove empty elements before calling querySelectorAll.
Update pseudoelements.html
Merge branch 'patch-1' of github.com:bensho/html2canvas into bensho-patch-1
0.5.0 rewrite
Make image loading to work on top of Promises/polyfill
Filter nodes and render correct size correctly
Cache backgroundImages request for nodes
Begin implementing background-image rendering
Add work around for chrome related background-image issues in iframe
Implement background rendering
Add Promise polyfill
Draw <img> elements
Add option to only render current view
Increase logging
Fix IE issues
ios window size and scroll fixes
Move NodeParser to seperate file
Fix text rendering for Opera/IE
Add font metrics and text-decorations
Add support for single text-shadow