Added basic embed.ly api response caching.

Regarding issue #46: Took some of the php code I wrote for your GoogleDocs2Twitter thing and built a rudimentary cache script, that stores the json requests as json files in a folder. You can call the embed-cache.php without any parameter using a cronjob to get rid of unused cache files after e.g. 30 days.
Maybe that’s not the best solution here, but it at least should get down the number of embed.ly api requests for the moment. Anyway, I would give it a test run before implementing it on your production server.