Added "forum_name" as a replacement code

Added {forum_name} to the topic reply email. {forum_name} will be replaced with the corresponding forum name (based on $forum_id) in the customized topic email. It's annoying that this info isn't included on sites with many forums.
use bbp_get_forum_title instead of bp_get_forum_name
Merge pull request #6 from arippberger/master

Added "forum_name" as a replacement code