Create product_price model.
Add has_many association of product_prices to product model.
Create product_application scaffold. issue #9
Create product_cross_reference model. issue #4
Add has_many association of product_cross_references to product model.
Create paper scaffold. issue #7
Create product_citation model for the association between paper model and product model. issue #13
Add associations between paper model and product model through citation model.
Create product_figure scaffold. issue #8
Create taxonomy scaffold. issue #10
Add "taxonomy" to ProductCrossReference::CATEGORIES.