Document Fields.hs

Documents most of Fields.hs. The generated output is available here: http://maxgabriel.github.io/yesodFields/Yesod.Form.Fields.html

For some field functions, the documentation doesn't contribute much (e.g. `textField`: "Creates a input with `type="text"`."), but I was pretty happy with others, which answered questions that before pretty much required reading the source and/or inspecting the generated html:

* `multiEmailField`—how is this achieved? (Personally, had never heard of the `multiple` attribute before)
* `emailField`—how are emails validated?
* `htmlField`—to what extent, if any, is the HTML validated?
* `Textarea`—what's the motivation behind changing newlines into `<br>` tags, and when does this occur?

NB: The exact documentation for `timeField` will depend on #874.