Replace stringTrimRegexp with a shorter and faster variant

1) Don't use unnecessary captures.
2) Use character classes instead of alternations.
3) Replace \u00a0 with \xa0 (saves 4 bytes).

According to http://jsperf.com/char-class-vs-alt this is 55% faster in IE7, 61% faster in Chrome 19, and a whopping 1840% faster in Firefox 11.