Update src/manipulation.js

It code bring leak of memory (test in chrome 23.0.1271.97 m, windows 7). Because every call $('selector').text('any text'), create 2 DOM nodes, but .empty() can't remove they. Code for test http://jsfiddle.net/3RSWf/ (I tested with source from repository and v1.8.3) and result http://my.jetscreenshot.com/7184/20130106-r5yo-219kb . It pull request fix problem.