Added support for publishTarget parameter, error handling bugfix

Now you can add publishTarget: 'trustedTesters' to account configuration to push to testers only (see https://developer.chrome.com/webstore/webstore_api/items/publish). Also when current version in store == version you attempt to upload error handler crashed, have fixed.
Merge pull request #6 from abelozerov/patch-1

Added support for publishTarget parameter, error handling bugfix