Feature/ Async storage + Google Drive example

this changes the storage class to work with Async storage providers,
and implements dos providers:
  - LocalStorage
  - Google Drive.
two questions here: 
- shouldn't plugin config be in separate files? (the idea is that people can chose not to have/configure some plugins right?)
- GoogleDrive.clientId is the same for all copay users? this should only be changed when deploying a custom instance of copay right?