Add Coinbase integration.

Fix order parsing.
Enable auto-redirect to always get return information.
PoC: Coinbase return processing

Add return test.

Rename parameter.

Revert "Rename parameter."

This reverts commit 87db691886bfdec15aacd9501a829291c60a16d8.

Rename post_params in return test.

Add return test to module tests.
Fix invalid acks
Also compare total_native
Merge pull request #62 from Shopify/coinbase

Coinbase