Added optionsPersistKey to Fields

There is no immediately obvious way to use optionsPersist with selectField. The function optionsPersistKey makes this possible. Example of use:
areq (selectField $ optionsPersistKey [] [Asc UserIdent] userIdent) "User" Nothing
Update Fields.hs
Merge branch 'master' of https://github.com/andrewthad/yesod
Minor tweaks for #590