Added new tasks to update entities, contracts and tenders.
Increased timeout of crawler.
Improved how last base_id is computed in crawler.

It now returns the last base_id from files or from db.
Improved crawler of tenders to be more robust to null dates.
Added migrations (Django 1.7).
Changed Tender description from CharField to TextField.

This was causing a tender with long description to not be inserted.
Fixed issue in storing dre url of tenders.

Tender now only stores an URL. For that we had to perform a
schema migration followed by a data migration.