Add mail option "failure or change"

[kwerle]

We would like to receive build notifications for all failures and changes to success. Right now buildbot supports either of those, but not both.

One of:

• all - Send mail about all builds, both passing and failing
• change - Only send mail about builds which change status
• failing - Only send mail about builds which fail
• passing - Only send mail about builds which succeed
• problem - Only send mail about a build which failed when the previous build has passed. If your builds usually pass, then this will only send mail when a problem occurs.

please add

• change or failing - Send email on all failing builds and also the change to success