Added a twitter search feed

Enables to create rss feeds out of twitter searches.
Ex: http://path/to/twitter-json-to-rss.php?=%23twitter
(don't forget to urlencode the hashtag (= %23)
- added the pubdate tag
- added the image tag (only if an image is included in the tweet).
Added instructions in the readme file
Fixed RSS for Validation + workaround for better facebook integration

- Improved RSS validation (pubDate + html content)
- If your rss feed is to be parsed by facebook, it will display twitter
default meta description as link description text... Not very useful.
Set the provided constant to true to enable a workaround using the
a_tweet.php file.
Merge pull request #3 from pixeline/master

added a twitter search functionality.