Move to Class + add Caching + return array or JSON

Hi there

As I've found you script really usefull, (now that twitter closed the 1.0 API), I've done some work to make it even more developer friendly.

Here are the main changes:
- Moved to a Class
- Cache ability (off by default, can cache either url requests or results)
- Returns Array (by default) or JSON
- Rename the script (class and files) to tweets2array
- Added a separate example file
- Updated the readMe (certainly need some fixes, I'm not english native...)

Hoppe you will like , and merge

Have a nice day