[FIX] openerp/osv/fields: fix method to_field() to pass some falsy parameters.

For instance, the attribute copy=False was not passed when converting the
_column to a Field.  Simply make sure those parameter are always passed.
[FIX] fields: in to_column(), returning self.column is generally not correct

Consider the following example:

    class Foo(models.Model):
        _name = 'foo'
        _columns = {
            'state': fields.selection([('a', 'A')]),
        }

    class Bar(models.Model):
        _inherit = 'foo'
        state = fields.Selection(selection_add=[('b', 'B')])

The attribute 'column' of the field does not have the full selection list,
therefore the column object cannot not be reused, even a copy of it.  The
solution is to systematically recreate the column from the field's final
specification, except for function fields that have no sensible way for being
recreated.
[FIX] purchase: do not pass uom in product_id_change when changing of product

To allow the on change to set the product default unit of measure when changing of product, and only in this case (not when changing price or quantity)
[ADD] actions documentation
[FIX] actions: match the behavior of multi actions with its help text

also fix the corresponding text and add explicit sequence number because I
don't understand what the bloody hell it does without that, except that it's
not the right thing. At all.
[IMP] new-api-ify 'trigger' server action
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32