Fourth test, over 7000 tweets scraped!

Searching backwards in history, several errors are caught:
HTTP Errors (by urllib2)
empty results
After these, the scraped tweets are safely stored and then exited.
Every tenth iteration, the tweets also get stored

Next step: scrape new tweets until previous most recent tweet (TestFive)
Added comments and a few minor changes

put the old search tests in comments at the bottom for future referral,
but cleans up the used methods for a clearer view

added a few comments for the implemented functionality