Store commit date and authoring date

Well, not really a bug, but something we really want for 2.0 IMHO.

I am starting a patch that would replicate the information on the {versioncontrol_operations}.date to have two fields: author_date and commiter_date.
Backends should modify the values later, on a hook_update_n().

Ok, here the first step: schema change and operation class data members change.

TODO: Change views implementation and default views.

Finally getting this ready.

I pushed the attached patch that:

    - Schema change on {versioncontrol_operations} to use two fields for the
      dates.
    - Change operation class data members accordingly.
    - Change views definition for operation dates.
    - Change a little how the versioncontrol_handler_field_operation_date
      views field handler work in order to support the handling of both
      author_date and comitter_date.
    - Change default views definitions on commitlog module to follow the new
      views fields, author_date and committer_date.

Update: I forgot to mention that we need to change default views on all dependent projects(i.e. versioncontrol_git, versioncontrol_project)

Just to mention the related tickets created: on versioncontrol_project #1203268: Change default views to follow versioncontrol api change on views data and on versioncontrol_git #1203264: Change default views to follow versioncontrol api change on views data

I 've just pushed a follow up to fix tests.