Fix bug in `removeClass`

Specifying a falsey value (such as the empty string) should have no
effect on the matched elements' classlists.
Complete function signature of `addClass`
Complete function signature of `removeClass`
Simplify `removeClass`
Extend iteration methods to accept nodes
Implement function signature of `append`/`prepend`

...additionally, ensure that the methods do not throw errors when
invoked with `undefined`.
Implement function signature of `after`
Implement function signature of `before`
Implement function signature of `replaceWith`
Prevent `find` from returning duplicate elements
Rename and simplify helper function

Because this helper does not create Cheerio objects, `makeCheerioArray`
is somewhat misleading. By instead declaring it as a recursive function,
it can operate on arrays and other objects, which obviates the need to
wrap arguments in array literals before invoking.
Merge pull request #290 from jugglinmike/manipulate-func-sig

Implement function signature for manipulation methods
Merge pull request #288 from jugglinmike/css-class-methods

Improve Css class methods