Match embedly API calls

I had a couple issues using this library. It may be that Embedly changed their API response.

1. The "media" type incorrectly lists "HTML" as an "int", but it should be a string. The tests pass because the mock response doesn't have a "media" response included. I added one from a Giphy query.
1. Embedly's API does not return an array of responses back when you pass in a "url" vs "urls". When is passes back a single JSON object, it fails to Unmarshal into []Response. I simply removed the "url" option for simplicity.
1. I also added a test to pull from the Embedly API if you supply your key as an environment variable. This would be nice to quickly sanity check the response from Embedly if it changes in the future.