configurable Sync strategies for models + collections

[funkytek]

looking to create alternative sync implementations (localstorage, firebase etc) and need ability and a consistent strategy. I think this merits some discussion since it spans models & collections and there are varying layers of abstraction

• ampersand-model encapsulates fetch, destroy, sync like backbone but it's sync implementation is hard-coded - need to be able to inject a sync strategy
• ampersand-collection does not have fetch etc, this exists rather in collection-rest-mixin but it also hard-codes it's sync strategy (will open an issue on collection)