Bump DB version 6 to 7

And fix update script
Remove ability to migrate from version before version 6 because we drop support of SQLObject. And at one point we can't provide support for it (Python3)
Add coveralls support
Add coveralls in README
Merge pull request #42 from bport/coveralls

Add coveralls support
Change the way we connect to the databse
Merge pull request #41 from bport/sqla-connect

Change the way we connect to the databse
Set python2 executable for update script and check if sqlo is available
Modify update script comments and text to be more comprehensive
Fix check on table exist
Merge pull request #40 from bport/sqla-bump-version

Bump DB version 6 to 7
Merge pull request #38 from agateau/sqla

Replace SQLObject by SQL Alchemy
Ran 2to3
Fix Python3 problem after conversion

- yokadi can be launched
- unit test state is ok
Fix Python3 stuff after merging