Update already imported products on calling update catalog #1981 fixes #69

Allow updation of products when the catalog update is called and the product
already exists in OpenERP.
Also allow updation of products from websites.
This allows user to choose which website info will be used a the principal
value.
Merge pull request #77 from shalabhaggarwal/feature/issue-69

Update already imported products on calling update catalog #1981 fixes #69