Record version numbers of tested dbs

Recording version numbers of tested DBs make it possible to mantain different versions of a software, and identifying regression bugs.

Version numbers can consist of:
* branch (that could be major.minor version number)
* revision (that could match to bzr revision or last number in release numbers)

In order to inform STK/Unit about the current version number, tested DBs can implement 3 optional functions:
* MEDIUMINT UNSIGNED get_branch_id()
* MEDIUMINT UNSIGNED get_revision_id()
Those functions provide numbers and is suitable for STK/Unit, that will use them to order/compare versions.

If those functions are there, 2 optional tables may be used to provide metainfo about branches and revisions:

branch_status:
* id
* name

branch:
* id
* name
* status_id

revision:
* id
* branch_id

PURPOSE
=======

The most obvious feature will be the ability to see the test results for:
* a given revision
* latest revisions for a given branch
* latest revisions for branches having a given status (ie: branches being still mantained)
* latest revisions for all existing branches

Some views will make these queries easier.

A less obvious feature will be the ability to identify possible regression bugs (failures or exceptions which were in x.x, then were fixed, but then came up again on x.y).