Fix exception after changing keypair

Two changes here:
* Rename `@items` to `@itemsCollection` to make `item_router.coffee` more testable in Jasmine.
* Remove the `||=` in `item_router.coffee` to make sure the new keypair gets set properly when the user changes keypairs. Tests included.

Renaming the collection is more to make it easier to test the router in isolation.  Without that change the function was getting clobbered by the property in Jasmine. 

The blank page issue also mentioned in #66 is still there but appears to be unrelated to this exception.

My experience with Jasmine is pretty limited so let me know if you'd like anything done differently.