Support creating artists.
Merge pull request #12 from starsirius/master

@dblock Support creating artists.
Move sale_artwork to a separate api/model.
Merge pull request #13 from starsirius/master

@dblock Move sale_artwork to a separate api/model.
Add delayed_task for rebuilding related sales and features of an artwork.
Merge pull request #14 from starsirius/delayed-tasks

@dblock Add delayed_task for rebuilding related sales and features of an artwork
alphabetize requires
Add ability to retrieve invoice by token
Add create_invoice_payment method