Added custom user agent to solve 429 errors.

I used your script in a bash script to download pictures from various
subreddits and encountered lots of 429 HTTP errors. With these changes
it ran smoothly.

From https://github.com/reddit/reddit/wiki/API:
Many default User-Agents (like "Python/urllib" or "Java") are drastically
limited to encourage unique and descriptive user-agent strings.

More information:
http://stackoverflow.com/questions/13213048/urllib2-http-error-429