[#381] Add Timestamp mixin

Add TimestampMixin abstract models class to use as mixin for models
needing timestamp fields for creation and last modified date.
[#381] Add TimestampsMixin to Organisation, Project and ProjectUpdate

Add TimestampsMixin to the models to enable automatic time stamping of
both creation and last modified dates.
[#381] Add schema migration for the timestamp fields in Organisation,
Project and ProjectUpdate.

Note that this is only a re-naming of the existing timestamp fields in
ProjectUpdate.
[#381] Refactor use of ProjectUpdate.time to ProjectUpdate.created_at

Find all uses of ProjectUpdate.time and change to
ProjectUpdate.created_at.
[#436] Add ProjectUpdate.user_agent field

The user_agent field is to record the device/user agent used when
creating updates. This is mainly to track the kind of mobile phone used
when bugs show up.
Merge branch 'feature/381_timestamp_fields' into feature/445_avoid_duplicated_updates
[#445]Add uuid field to db
[#445]include DB migration script
[#445]Merge branch 'feature/445_avoid_duplicated_updates' into develop

Conflicts:
	akvo/rsr/models.py