Improvement on product_margin

* remove sale & purchase dependacy
* graph view for turnover , total cost
* add a sum="..." on the fields: turnover, total margin, sales gap
in the tree
* By default the wizard propose to report on the latest year. (not empty
fields)

The title of the tab must be Margins instead of Margin. Put tooltips on
all fields that explain how they are computed.
* made new form view and set high priority
* modify states : (paid,open_paid,draft_open_paid)

bzr revid: hmo@tinyerp.com-20081010122744-5sw8yfbopt0zr04k