Improve the behavior of the Reciever and node logo file upload

* File upload starts automatically when file is selected.
* Progress bar is displayed to warn the user of progress.
This fixes: https://github.com/globaleaks/GlobaLeaks/issues/419
Fix some bugs in the progress bar visualization
Merge pull request #76 from globaleaks/issues/205

Improve the behavior of the Reciever and node logo file upload