Move store credits out

[IrvinFan]

This is the starting point of moving store credits feature out to Spree. It's a result of the paired coding exercise I had with @paulo-surfdome.

Here is the approach we took: go through codes in the directories and copy (new file) or merge (decorator) store credits related codes into Spree (Related PR: https://github.com/spree/spree/pull/5625). And at the same time remove those codes from the extension (this PR).

Right now the key components are done. Anyone can take from here.

• [x] merge app directory
• [x] merge bin directory
• [x] merge db directory
• [x] merge config directory
• [x] merge lib directory
• [x] merge spec directory
• [ ] rename this extension