Fixes for @document blocks

[tophf]

• Fix line, col in "enddocument" event.
• Support @keyframes inside @document blocks.
• Support nested @document blocks: @document inside @document.
• Fix @keyframes: consume trailing whitespace after RBRACE
  All other block parsers do it, but _keyframes() didn't, consequently breaking the parser after @keyframes inside @document blocks.

[jawshooah]

Might want to add a testDocumentWithKeyframes (example) to tests/css/Parser.js (and tests/css/CSSParserTests.htm, assuming that's still in use).

[tophf]

Added, thank you!

[cscott]

@XhmikosR this patch set can be directly merged as well. LGTM.

[cscott]

@frvge Perhaps you could review this one? It looks good to me.