[MERGE] [REF] project task/issue stage management cleaning

[MERGE] [REF] project task/issue stage management cleaning

Using stages and states is not easy to manage. Indeed those two fields mess with the same basic concept. This task remove the state on project.task and project.issue models, leaving only stages.

Main features:
- removed base_stage inheritance on project.task and project.issue models
- removed state field on project.task and project.issue models
- removed state field on project.task.type model
- added date_last_stage_update on project.task and project.issue models, holding the date of the last stage change, allowing reporting and analysis on time spend in stages
- date_open is set when assigning an user to the task / isuse; date_open now means assignation date

This allows to remove all buttons on the form view, leaving only a dynamic stage management using the statusbar.

Subtypes have been updated :
- new: stage_id.sequence=1
- stage changed: stage_id changed, not new
- user assigned: when user_id is set

Misc:
- [IMP] views accordingly;
- [IMP] reports accordingly, added date_last_stage_update in reports;
- [IMP] subtypes
- [REM] [REF] [TESTS] removed some tests because they are not applicable anymore; rewrite some tests and moved some of them into unittests
- [DOC] doc directory + changelog + stage explanation in project and project_issue;
- [REM] removed project.task.reevaluate wizard

bzr revid: tde@openerp.com-20130719094706-ea0hbwjj99pochzo