Option to remove revision log field

This patch adds three options for removing the revision log field:

* Enabled (if revisions are enabled)
* Only show when user has View Revisions permission
* Disabled (for all users)

The patch uses the #access attribute on the revision_information node of the form object. This is a good way to hide stuff because it's still available to the submit handler, however it doesn't actually get hidden for user 1 (because user_access() always returns true for them).

After spending years telling users to "just ignore that field" for the revision log field, it would be GREAT to have a module that could hide it.

I'm really enjoying this module, btw!