Use ActionDispatch::Request#remote_ip instead of reinvent the wheel

http://apidock.com/rails/ActionDispatch/Request/remote_ip

Fixes #4490
Fix issue changing between payment methods in admin

Fixes #4358
Fixes #4203
Payment credit_allowed deducts from offsets

fixes #4403
Fixes #4407
ShippingMethod#shipments should look up shipments through linked shipping rates

It used to be that shipments linked directly to shipping methods, but now the relationship is done through shipping rates.

Fixes #4492
Bump to 2.0.10
[api] Redirect to Orders#show from Checkouts#show

Rather than having two actions that do the same thing, let's just combine all the logic into one.

API users: please use /orders/:num from this point onwards.
Bump to 2.0.11.beta