[FIX] removes non stored fields from measures

Non stored functional fields cannot be grouped by read_group, so it is
useless, and even harmful, to display them in the measure list (in
graph view).
[IMP] better and shorter contribution guidelines (detailed one are now on the wiki)
remove useless quotation marks
Merge pull request #521 from odoo-dev/master-graphview-measures-fix-ged

[FIX] graph/pivot tables: remove non stored fields from available measures
Merge remote-tracking branch 'odoo/master' into master-default-order-list-ged
[IMP] improves sorting mechanisms in datasets

Create a deserialize_sort method and add a set_sort method to
datasets.  The set_sort method is useful to avoid duplicating code
in lists and kanban views (to set the initial default order)