Add search-by-string on key function

[doublejosh]

Allows searching items by key using a string.

$.jStorage.search('my-namespace-values');

[doublejosh]

Needs testing, adapted from my current addon, related to this issue... #52

[andris9]

This approach assumes that jQuery is present while jStorage does not. Would it be possible to do the same thing but without $.extend (and also add a test for it here)?

There's also an awesome fulltext search library for js called lunr.js but I guess for this specific task it would be an overkill.

[doublejosh]

Zepto has extend in it's core too. Are there other candidate dependencies?