Fixed corner case in trends metric.
Merge branch 'master-patch'
[scm] Add Revisions metric

This metric is a subset of the Commits metric. At some point those should
be integrated into the same class. This class counts unique revisions across
all of the repositories.
First try at querying TZ for the authors data in SCM.
Some cleanup and code to filter out some developers.
Added create_report for Ages object. Seems to work with report_tool.py.
Ages_ITS now has its own report generator (but I canot make automator to call it).
Aging analysis now seems to work pretty well both with ITS and SCM.
Some clean-up and minor modifications.
Timezone analysis. Seems to work for SCM.
Now mls_query.py seems to work...
Some restructuring, common parts to common_query.py
A bit of restructuring of the GrimoireDatabase hierarchy.
Starting restructure of ActivityPersons classes.
Demopgraphy analysis for MLS seems to work...
After some minor modifications, ages.py seems to work with MLS now, too.
Rearchitecturing of scm_query done. This should work...
Nos, scm.py and related tests seem to work with the new architecture for
scm_query.py.
Including calculus of variables using sessions.
New method get_session() to obtain the session of a SCM object.