Support "use strict" mode in phono.js

[andrewdeandrade]

This library causes my browser to barf with a global "use strict" statement, such as the one automatically inserted by uglify2. Strict mode also gives better error handling and permits certain code optimizations.

mishoo/UglifyJS#128
http://www.nczonline.net/blog/2012/03/13/its-time-to-start-using-javascript-strict-mode/