Fix implementation of $.fn.children

As per @davidchambers's feedback:

- use `_.reduce` rather than a for loop
- pass `isTag` to `_....

According to [the jQuery documentation on
`$.fn.children`](http://api.jquery.com/children/):

> ...

Introduce "$.contains"

Remove global `render`

Cancel iteration when iterator returns false

As noted in the [jQuery API docs on
.each()](http://api.jquery.com/each/):

> We can stop the lo...

From [the jQuery API docs on
$.contain()](http://api.jquery.com/jQuery.contains/)

> Description...

use a bound function rather than passing context to `_.filter`

$.find should query descendants only

Find should not query the element it is called on. For instance, the
following should return 1:
...

use `_.each` and `_.map` to simplify cheerio namesakes

Added a toString() method

Added filter() with tests and updated readme

removeClass removes spaces

removed reference to size() from the readme

Minor changes to placate jslint.

squashed commits for issue 105 - test for numeric attributes

manipulation: DRY out `append` and `prepend`

Add syntax highlighting to readme

Regression in removeClass (null ref)