This PR adds the "abandoned" state to the RPC call listtransaction (only outgoing transactions), it also adds a context menu option to abandon a transaction over the GUI.
Abandoned transaction are marked with a light-red color and a "trash" icon.
Some screens: <img width="1056" alt="bildschirmfoto 2016-03-17 um 20 04 17" src="https://cloud.githubusercontent.com/assets/178464/13858483/32d94ed4-ec7f-11e5-8909-59df5b245a5a.png"> <img width="732" alt="bildschirmfoto 2016-03-17 um 20 04 09" src="https://cloud.githubusercontent.com/assets/178464/13858486/34038a54-ec7f-11e5-9dc0-70b255a5be40.png"> <img width="319" alt="bildschirmfoto 2016-03-17 um 20 31 17" src="https://cloud.githubusercontent.com/assets/178464/13858496/38a1a5c8-ec7f-11e5-8f72-767b69f233d7.png">