Fixed: incorrect parameters passed to webmock response factory.
Fixed: loosened test assertions in the large gem list test to ignore
additional gems fetched by geminabox itself.
Moved dest_filename from IncomingGem to GemStore class.  Better
incapsulation - gem doesn't need to know about the details of how it
will be stored.
Made Geminabox Gem store object configurable through Geminabox.store
Moved gem delete functionality to the gem store.