Move doc parsing out of JsParser & CssParser.

Instead do it in SourceFile which bind it all together with Merger.

This move brought out some glaring errors in previous benchmarks -
the JsParser also did doc-parsing while EsprimaParser did none of it.
Now things are much more equal and EsprimaParser receives a larger
performance penalty.

Again, parsing full Sencha Touch sources:

JsParser:      2.9s
EsprimaParser: 4.5s