New feature: use sqlalchemy

The most important changes:

* setup.py: Bump to 2.0 version because there are substantial changes
that affect major aspects of stats_consolidation usage.

* stats_consolidation/db.py: Uses SQLAlchemy library to interact with
database. This allows to change database driver to any supported by
SQLAlchemy itself.

  Some changes are due to portability issues:

  - change table name to lower case
  - conversion to Unix epoch in Python
  - use appropriate types in queries

* sql/make_report: Add --db_dialect optional argument.

* stats_consolidation/stats_consolidation_run: Add an optional
configuration entry db_dialect.


Please, tag with v2.0.0.