Update to post-then-commit flow for purchases

Just like return invoices currently do in the 2-2-dev branch.

Create an Avatax "SalesOrder" to capture tax info prior to the "confirm" step.
At the "confirm" step create an uncommitted Avatax "SalesInvoice" to capture tax information.
Then after the order is complete commit the existing SalesInvoice.

Also:

- Provide a DB migration to create uncommitted SalesInvoice records for existing orders in the "confirm" state
- Proivde a rake task to backfill SalesInvoices for historical orders that have been sent to Avatax
- Remove now-unused "avalara" gem
- Remove obsolete "endpoint" config
Update Readme
Merge branch '2-2-stable' into 2-2-dev

Conflicts:
	README.md
	lib/generators/spree_avatax/install/templates/config/initializers/avatax.rb
	lib/spree_avatax/factories.rb
	spec/spec_helper.rb