Expanded testing of _.escape() and _.unescape() to cover all conditions

Tests for _.escape() and _.unescape() did not cover all the possible escapable strings, and did not test for multiple replacements of the same substring (e.g. 'Larry & Curly & Moe')

These extra tests are the only thing left from my ill-fated attempt at improving performance of the escape/unescape methods by testing for the necessity of changes using indexOf before escaping ([indexOf is very cheap compared to the regex replace](http://jsperf.com/regexp-vs-indexof))