Improvements for imagewriter

I propose some improvements:
- replaced the dd command with direct writing command python.
- progressbar is functionnal.
- added writing ~/.imagewriter.log at the end.
- the application doesn't close when a stick is inserted after the start and a dialog box which is displayed to saye it.
- point at the end of the logview window.
- indicating the name of the log file.

My start point was the Ubuntu src. I found this one later. The difference with linuxmint one is the usage of the banner, some modifications in the .glade file that I did not understand, and the find_devices.
Can be linuxmint interested with that?