Correct bug in implementation of `Cheerio#val`

jQuery does not attempt to calculate the value of a `radio` input group.
Instead, when the value of such an element is queries, that element's
`value` attribute is returned directly.

Although slightly less convenient in some cases, this allows for direct
manipulation of `radio` input element values (which is otherwise
impossible).
Fix bug in `css` method

When a callback function is provided as the second argument to the `css`
method, it should be invoked with each element's index and the value of
the specified CSS property (not the element itself).

From the jQuery API documentation on `css`:

> # .css( propertyName, function )
>
> **propertyName**
> Type: String
> A CSS property name.
>
> **function**
> Type: Function( Integer index, String value ) => String or Number
> A function returning the value to set. `this` is the current element.
> Receives the index position of the element in the set and the old
> value as arguments.

http://api.jquery.com/css/#css-propertyName-function
Remove unofficial signature of `children`

jQuery's implementation of the `children` method accepts an optional
selector--it does not treat Number values differently.

From the jQuery API documentation of `children`:

> # .children( [selector ] )
>
> Description: Get the children of each element in the set of matched
> elements, optionally filtered by a selector.

http://api.jquery.com/children/
pass options to $.html in toString

fixes #611
remove unnecessarily tricky code
Merge pull request #616 from davidchambers/straightforward

remove unnecessarily tricky code
Merge pull request #583 from jugglinmike/fix-css

Fix bug in `css` method
Merge pull request #544 from jugglinmike/fix-val

Correct bug in implementation of `Cheerio#val`
Merge pull request #584 from jugglinmike/fix-children

Remove unofficial signature of `children`
#608 -- Add support for insertBefore/insertAfter syntax. Supports target types of: $, [$], selector (both single and multiple results)