Optimize and simplify escape

Escape is optimized by testing whether the string contains some escape character prior to replacement.

Some discussion https://github.com/megawac/underscore/compare/escape-simp

JSPerf: http://jsperf.com/underscore-escape
The other option (used by lodash since https://github.com/lodash/lodash/commit/66180b14c54c040ff80a519b79d47e4cf5bd9c0f#diff-001d0647fb00f8336795faccdec19a31R7816) is to set `replaceRegexp.lastIndex = 0` and test with the global regex