improve parser performance

[alexlamsl]

Measured ~5% speed improvement on es6.html when running benchmark.js

[alexlamsl]

The diff on github makes it look really dramatic, but in summary what I did:

• moved html.indexOf('<') from text handling forward, as all the other blocks depends on < being the foremost character to be processed, thus saving the transversal through all the case mismatches when text blocks are encountered
• record and update last tag name instead of repeatedly calling stack.last(), also prevents modifying the array object prototype which may have performance penalties

[kangax]

Cool, feel free to merge (once it's rebased). As long as all tests are passing and code doesn't look too cryptic, I'm ok :D