use correct stores endpoint in mock server
add a purchases mock endpoint
add a purchase model
add a model that knows how to perform checkouts
provide mechanism for checking out in view
hook up checkout in template
people cannot buy more than they can afford
'customers-can-check-out

* customers-can-check-out:
  people cannot buy more than they can afford
  hook up checkout in template
  provide mechanism for checking out in view
  add a model that knows how to perform checkouts
  add a purchase model
  add a purchases mock endpoint
  use correct stores endpoint in mock server
version bump: 0.1.0