Document the Options section of Fields.hs

Haddocks available here: http://maxgabriel.github.io/yesodFieldsOptions/Yesod.Form.Fields.html

This documentation was somewhat tricky. All of the Options field functions have a very similar structure, so instead of documenting each one in detail, I gave a high-level overview of how `Option` and `OptionList` worked. 

Example code is given for the first two options field functions and for the `optionsPersist` function.