Many changes:

* Add support for following users
    + Must use PIN authentication and an application that has
      read/write permissions
* Search was broken because Twitter (recently?) can return fewer
  results than requested, which was counter to my assumptions.
  Fix is a little more complex than necessary because search and
  timeline browsing share the same code path, but the end result
  is a little more explicit and a little less redundant code.
* Fix is_retweet/1 (was exactly wrong)
* Add some -specs
* Significantly slow down oembed requests because Twitter only
  permits 180 for each 15 minute block. Need to make this much
  smarter because it's atypical to need more than that for any
  given set of requests.
* Change oembed_search to oembed_tweets because it's silly to do
  both operations at same time.
* Add HTML wrapper around embedded tweets.
* Add rate_status() to check for rate limiting.