Provide convenience functions for CRUD of custom resources

[obmarg]

#56 added custom resource support to kazan. However, CRUD requests on custom resources require a user to manually build a Kazan.Request type. It'd be good to provide some utility functions for building these requests.

I'm thinking something along the lines of:

defmodule Kazan.CustomResources do
  @spec create(struct) :: Kazan.Request
  @spec delete(struct) :: Kazan.Request
  # Etc.
end