Fix "\\{{" immediately following "\{{"

Escaped-escape mustaches ("\\{{") immediately following escaped
mustaches ("\{{") were being handled incorrectly.

Fix the lookahead to make sure yytext still contains the appropriate
slashes when we pop out of <emu> so they can be handled consistently
by the initial state.
Merge branch 'fix-escapes' of github.com:dmarcotte/handlebars.js into dmarcotte-fix-escapes

Conflicts:
	spec/tokenizer.js