BUILDBOT: Add "other" build set.

This is intended to be used to test extensive changes i.e. Pull Requests
which could impact across ports and should be tested before merging.
BUILDBOT: Switch "other" buildset to gsoc2012-scalars-cont branch.

This has been outstanding for some time due to issues about whether
various ports are broken.
BUILDBOT: Disable stable buildset and add note on buildset disk space.

Only two can be enabled at any time plus tools. This commits switches
to tools-master, master and other only.