Adding custom user agent.

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.

Look here for more information:
http://stackoverflow.com/questions/13213048/urllib2-http-error-429
Merge pull request #9 from whatevsz/master

Added custom user agent to solve 429 errors.