Improve performance when Twitter is unavailable

When Twitter is offline, the module always makes attempts to get recent tweets and return old ones if fails to do so.
However, the timeout of 2 seconds is still too big for high-traffic sites (2 seconds of Apache cost really much in busy environments).
A workaround to this is doing requests less frequently, if Twitter is down.

Note: all that makes sense only if tweets are retrieved server-side (not via an Ajax request), using twitter_pull_retrieve function.

Patch pending...