Move store credits in core

This is the starting point of merging store credits feature (but leave gift cards feature) from extension https://github.com/paulo-surfdome/spree_store_credit_payment_method/tree/move_credit_out into spree core. Anyone can take from here.

The following directories are done:
app
bin
db

The following directories need to be worked on:
config
lib
spec

Additionally need to implement store frontend UI to really leverage this feature.