Only load wp-admin/includes/misc.php when needed
Remove unnecessary file include
Remove dead options

No need to pollute a user's database with options that are no longer used.
Improve loading of group-fourm-attachments.php

We only want to load group-fourm-attachments.php under two conditions:

1) Legacy forums are enabled
2) The site has been running Group Documents for a long time and set up forum attachments

We don't want add an extra db query to get an option that may not be available (see previous commit 0f34d961).
Update version numbers to 2.0-beta1