add some documentation for streams

See also questions and discussions on this topic:
http://stackoverflow.com/a/17536438/859591
https://dev.twitter.com/discussions/8081
https://dev.twitter.com/discussions/8110
Import TwitterHTTPError for stream module
Merge pull request #174 from RouxRC/master

POST for "statuses/filter" in Streaming API
Twitter recommends to use preferably POST for the filter method in the Streaming API https://dev.twitter.com/docs/api/1.1/post/statuses/filter
So it should be listed here
Merge pull request #167 from lumbric/master

Add stream documenation
It was very difficult to find information on this topic. Now that I figured out how to get direct messages, I added it to the README.

See also questions and discussions on this topic:
http://stackoverflow.com/a/17536438/859591
https://dev.twitter.com/discussions/8081
https://dev.twitter.com/discussions/8110
Merge pull request #168 from mjumbewu/patch-1

Import TwitterHTTPError for stream module
The TwitterJSONIter raises TwitterHTTPError on HTTPError, but it is never imported into the module.