define getSyncMethod locally so it doesn't get clobbered by other Backbone modules

I'm using Backbone.dropboxDatastore and Backbone.localStorage on the same page. They both define `Backbone.getSyncMethod` which causes `Backbone.sync` to enter an infinite loop. This patch makes `getSyncMethod` a local function so that doesn't happen. A similar patch may need to be applied to Backbone.localStorage (I haven't tried without also making the change there yet).