Clean up imports

Change-Id: Iba19f80402c2574ccc492c4246c0b70dd0d2f901
Do argument parsing in main

Change-Id: Id0ba991580b5271b4d91897842ffd4b4a456732f
Allow not overriding existing files and send appropriate user agent

Change-Id: Ibfbb4fede0b93d29f580580e75bbe525cc388848
Better filename choices

Change-Id: Iae1247cd95591461fceaa89f4513c387cc0bb216