Moved some methods to private closures
Moved some methods to private closures

For performance and compressability, i created a private closure scope
in the Collection class and moved some of the underscore methods into
private closure functions.

YuriSolovyov/Dexie.js,jacobmarshall/ng-dexie,dfahlander/Dexie.js,cesarmarinhorj/Dexie.js,jacobmarshall/ng-dexie,chrahunt/Dexie.js,chrahunt/Dexie.js,jimmywarting/Dexie.js,dfahlander/Dexie.js,chrahunt/Dexie.js,cesarmarinhorj/Dexie.js,jimmywarting/Dexie.js,dfahlander/Dexie.js,dfahlander/Dexie.js,jimmywarting/Dexie.js,YuriSolovyov/Dexie.js,jimmywarting/Dexie.js,chrahunt/Dexie.js