Fix Traceur on the speed comparison page.

https://code.google.com/p/esprima/issues/detail?id=424
Simplify regular expression scanning.

To reduce the complexity, split it into body and flags scanning.
Also, as suggested by Bei Zhang, separate the testing of regular
expression so that it's try catch won't reduce V8 performance.

https://code.google.com/p/esprima/issues/detail?id=396