Subscribe at comment level & autosubscribe feature

A working version of this patch can be seen at a drupal test site: www.pvun.com

This patch lets users subscribe to individual comments instead of entire nodes.
This way, users can choose not to be notified about comment threads within a
node that they are not participating in or interested in. They still retain
the option of getting notified about every comment added to the node. Each
comment now has a "subscribe" link so users can subscribe to comments they
are interested in.

Second, this patch adds a checkbox just above the "Preview" and "Post" buttons that
reads "Receive notification of replies to this comment/story." If the box
is checked, the user receives an e-mail when someone replies to the comment
or node. Note that for comments, this checkbox requires a small patch to the comment module which can be found here: http://drupal.org/files/issues/comment_9.patch.
The patch adds two lines of code to the comment.module. It is unclear at this
point if the patch will be accepted into the core.

The checkbox describe above is automatically checked if the user has
the "autosubscribe" checkbox checked off in their profile. The user can override
the "autosubscribe" setting by simply unchecking or unchecking the checkbox
for individual comments and nodes that he/she posts.

The "autosubscribe" setting can also be changed from the user's list of all subscriptions.