Remove RBE mail filter before sending bounce-back emails.

This ensures that these emails should not be allowed to be replied to.
For WP Better Emails support, adjust the position used to remove the
RBE marker.

See related commit b07f430.
Add additional checks for auto-replies.

Check if the 'Return-Path' contains either 'mailer-daemon' or 'owner-' in
it.  If so, this is an indicator of an auto-reply email, which we should
block from posting.
Introduce BP_Reply_By_Email_Admin::get_plugin_basename() method.

Extrapolates the inline code from the add_plugin_action_links() method.
This will be used a few other times, which is why it's being branched out
into its own method.
Support auto-updating RBE through GitHub.

Uses the Plugin Update Checker library by Janis Elsts:
https://github.com/YahnisElsts/plugin-update-checker

Licensed under the GPL License.
Update 'Thanks' list in readme.md. Also add 'Translation' section.
Update readme.txt.
Bump version number to 1.0-RC2.