Support escaped "]" in regexp literal character classes (ex: "/[\]/]/")

https://code.google.com/p/esprima/issues/detail?id=442

The RegExp expression parser did not handle the escape sequence "\]" within
character classes, or any escape sequence for that matter. Once in a character
class, the parser consumed all characters until reaching "]".

This diff makes the parser always consider escape sequences, whether it is
currently inside of a character class or not. This more closely matches the
grammar, which looks like:

```
  RegularExpressionChar ::=
    RegularExpressionNonTerminator but not \ or / or [
    RegularExpressionBackslashSequence
    RegularExpressionClass

  ...

  RegularExpressionClassChar ::=
    RegularExpressionNonTerminator but not ] or \
    RegularExpressionBackslashSequence
```

http://es5.github.io/#x7.8.5

That is, the parser can derive RegularExpressionBackslashSequence both inside
and outside of character classes.

Test plan: Added test case to test.js and loaded it in Chrome 30 and Firefox 25.
These browsers produce different results for `new RegExp('[\\]/]').toString()`
so the test case dynamically computes that value.