Use "loaded" instance of Cheerio in unit tests

This usage allows tests to be slightly more concise. In addition, it
better reflects the real-world usage of Cheerio, promoting the unit
tests' role as active usage examples.
Merge pull request #504 from jugglinmike/test-loaded-cheerio

Use "loaded" instance of Cheerio in unit tests
Implement `Cheerio#add`
add class only if doesn't exist
Merge pull request #502 from jugglinmike/add-add

Implement `Cheerio#add`
added test
Merge pull request #503 from alexindigo/master

Added options argument to .html() static method.
Change state definition to more readable
Merge pull request #505 from 0xBADC0FFEE/toggle-class

toggleClass: add class only if doesn't exist
Respect options on the element level