Call Callbacks and Attachment filename

[SunnySydeUp]

There are two commits:

1. The callbacks should be called regardless of the processVisibility flag in the postExecute method.
2. The attachment filename should be name of the file attached and not it's entire path.

[luongvo]

This PR will resolve issue #7 and #28 👍

[tushardhole]

@luongvo @yesidlazaro
Can we please get this merged ? I cannot see this changes available, in the latest version.

[luongvo]

@tushardhole please switch use my fork version https://github.com/luongvo/GmailBackground instead atm, seems the author is not active anymore on this repo