Update Tor Browser videos

Previously, we had videos for teaching users about downloading, verifying and running the Tor Browser. These videos are now quite out of date, as they were written for the 3.x line of Tor Browser releases. As part of Sponsor O, we are supposed to update these videos to reflect the current 4.x Tor Browser. Sherief has committed to working on these videos, and a tentative deadline has been set for Q1 2015. Once the videos are complete, a native English speaker at the Tor Project (likely Colin Childs) will need to handle the voiceover.