Allow Bamboo build number to be determined from Perforce or Subversion changeset/changelist

Ability to use the latest changelist/changeset as the buildnumber for the plan.
This makes the build plan key be much more informative about it's content,
since it's possible to just get the information from VCS based on the key name
as the changeset number.

CruiseControl has the P4ChangelistLabelIncrementer that is used for this purpose, and its much better to use it in perforce projects than the standard label incrementer, which increments each build by +1

P4ChangelistLabelIncrementer

"Creates a label from either a given Perforce changelist number, or the most recently submitted changelist. A changelist number can be a good choice for labeling builds from Perforce, because a changelist indicates a particular state of the depot at a moment of time which will never change."