0.4 release

- [X] improve db.use()
- [X] add schema.dropStore() and schema.dropIndex()
- [X] db.write([’st1’, ’st2']) - для использования одной транзакции
- [X] improve batch (reuse transaction)
- [X] onversionchange #16 
- [ ] improve tests
- [ ] improve docs for store.cursor (+lines of code, docs for schema.drop* methods, and transaction reuse)
- [ ] fix iOS 8 & Safari 7.0.6, IE10 & IE11 support (add this to Readme) #17