Allow value and text method options to radio_button_fieldset (#82)

[misaka]

NOTE: There are two changes here, one changes the existing radio_buttons_fieldset and the other adds a collection_radio_buttons method. See the discussion happening in issue #82, my proposal is that we just make the change to radio_buttons_fieldset for now and drop collection_radio_buttons until we can do it properly.

When generating a radio-button fieldset, there are times when we want to specify
how to get the form value or label text for the choices passed in, the same way
collection_radio_buttons does in Rails. For example, when assigning a team to
a person we may want to use the team's DB ID as the value and the team's name as
the radio button text. This change adds these options to
radio_button_fieldset (adding them to other places, such as
check_box_fieldset, is an exercise for later).

This addresses issue #82.

[misaka]

@aliuk2012 Just to check, this PR has both changes in it, the addition of value_method and text_method to radio_buttons_fieldset AND the addition of a new method collection_radio_buttons. Are you approving the addition of both of these?

[aliuk2012]

Could you strip out the collection_radio_button method and move that into its own branch? If we log an issue and reference that branch we can then look at adding that method as part of another feature.

[stephenrichards]

Agreed, lets get radio_button_fieldset merged, and put the collection_radio_buttons into a separate PR for further discussion.

[misaka]

Commit removed, merging.