check for invalid api key message

When json.loads() raises a ValueError, the reason may be that
the submitted FCC API Key was invalid. The FCC returns a small
html page titled "Invalid Key". This change checks for that
string in the response text and raises a more useful error
than a json parsing failure. If that text is not present, it
raises the original json parsing ValueError.
Merge pull request #8 from selik/master

check for invalid api key message when json fails to parse