Handle API rate limiting

Pass `retry=True` when creating the Twitter instance to handle these two
cases automatically:

- On HTTP error 429, read the `X-Rate-Limit-Reset` HTTP header and retry
  after the specified time.

- On HTTP errors 502, 503, and 504, retry after 30 seconds.
Make the delay after 50x HTTP errors configurable
Add documentation for retry=True
Allow to set the number of retries
Merge pull request #253 from bbolli/limit-retry

[RFC] Handle API rate limiting