Postbox 1.1.5

* Several stability fixes
* Memory improvements to make Postbox give back memory more aggressively
* Performance improvements for sending and reading large messages and conversations
* Add the ability to control signatures in message replies and forwards from Tools | Account Settings... | <account> | Composition & Addressing
* Hide the folder pane when in a message tab
* The mechanism for logging into Twitter has changed. Twitter users will need to log in again via Tools | Web Services...
* Fix an issue with POP3 accounts where checking for new mail failed with an error about the folder is currently being processed
* Fix an issue where copying to the Sent folder would fail
* Fix an issue when dragging a message into the compose window was failing to attach the message
* Fixed an issue with IMAP offline stores larger than 4GB
* Fixed a conversation threading issue where messages were not threaded when they should have been