Gracefully handle empty lists of reddit friends

For issue #30

Also use HTTPS to login...
Log level is optional for 'global', 'user', or 'none'

default: --log user
log everything to one file: --log global
disable all logging: --log none

Closes #35
Updated default user agent
Fix scraping single-image imgur pages

Ugh.

For issue #38
Support for GIFV imgur content

Grabs .mp4 file. Closes #40