Make underscore.string a fully standalone library

As we are doing breaking changes on the 3.0.0 release anyway I want to take this opportunity to make underscore.string a fully standalone library.

This basically means an internal chaining implementation and documentation that discourages using it as an Underscore.js extension as I think it was mistake in first place because of so many name collisions with Underscore functions.

New readme can be previewed here: 
https://github.com/epeli/underscore.string/tree/standalone#underscorestring-

Changelog:
https://github.com/epeli/underscore.string/blob/standalone/CHANGELOG.markdown#300-unreleased

Any comments are welcome. Even just "+1" :)
Nah. Too early. index.js is pretty small and it's not used anywhere else.