The error callback will now be triggered if the server returns a non-200 HTTP status code.
_signedPost() now inspects JSON responses for error statuses, and passes the error message to the callback if necessary.
Sometimes, errors don't come packaged nicely as JSON. Sometimes they are HTML that looks like "<h1>401 Invalid Signature</h1>" or "<h1>403 Developer Inactive</h1>". We now handle those cases.
Merge pull request #8 from steveluscher/master

[node] The error callback should be triggered if the server returns a non-200 HTTP status code