Catch the standard Unix SIGTERM kill signal and do a graceful shutdown of PyBitmessage.

We already do this for the SIGINT kill signal. The change allows us to do a clean
shutdown of PyBitmessage when its process has been separated from the terminal.
Bitmessage Protocol Version Three
very minor improvements to ProtoV3
Normalize the noncetrialsperbyte and payloadlengthextrabytes for each of the user's existing addresses
Added ability to limit network transfer rate
After upgrade period, also support old v2 addresses
updated defaultKnownNodes
Merge pull request #709 from JonathanCoe/master

Catch the standard Unix SIGTERM kill signal and do a graceful shutdown
resolve minor merge conflict with master
save debug.log in correct directory
Revert "Catch the standard Unix SIGTERM kill signal and do a graceful shutdown"
Merge pull request #726 from Bitmessage/revert-709-master

Fix #725. Revert "Catch the standard Unix SIGTERM kill signal and do a...
Merge branch 'master' into ProtoV3
Bugfix: infinite loop if our max acceptable difficulty is too low
Merge pull request #727 from Bitmessage/ProtoV3

Proto v3