title: Gmail api - credential storing
body: <p>On the one-time process to get credentials for gmail api, a directory on file system must to be supplied to store them on a json file (DATA_STORE_DIR).<br>
  I'm using gmail api for Java and i don't have chance to use file system from my app to store that json file. 
  Is there any other way to store credentials (e.g. database)?
  Otherwise, should I provide my own impelmentation of FileDataStoreFactory class?
  Thanks.</p>