Support for Perforce changelogs in Continuum and changelog plugin

Continuum wasn't reporting author/date/comment for each build's modified files when used with Perforce. I tracked the root cause to the update command not returning the current changelist, which in turn wasn't being provided to the changelog command. At the same time, the changelog command didn't accept revision-based ranges, only dates.

The attached patch (with updated test cases) makes the Continuum build status pages return full results.

The basic flow is to get the newly-submitted changelist numbers using "p4 changes", then populate ChangeSet objects directly using "p4 describe".

A side effect is that this replaces the patch for SCM-332. The original code gathered the changed files (and their comments) with "p4 filelog", which the patch then re-aggregated into changesets. The new version accomplishes the same thing, but more directly.