Refactor of spree-multi-currency.

(cherry picked from commit 43ac0e29abb47829a3a91b406fca4f90ba69d813)
Ensure session[:currency] is still supported when returning current_currency
(cherry picked from commit 3d2a276dde57f121067616080bb282decb7edb81)
Changes needed to make spree_multi_currency Spree 2.0.0 compatible.
(cherry picked from commit 4eb85e7a17ad86b04b2948408f27ca189cdaec96)
Added dependency to spree backend.
(cherry picked from commit 090e1e523ab37e1026f8a1d0a0923951e2ff63d7)
Documented the installation with rails generator.
(cherry picked from commit fe8573d2acbd7589bc626bd347b8d4c013d34024)
Removed Spree Backend dependency.
(cherry picked from commit 6a5bb7014fbf27bb7b7d7dbf6c631a63903352a4)