verify only takes a single apikey to check

This updates the API so the user is required to pass in the key they
want to check.  If the user had added more than one key to the nma
client, the call to Verify() would fail because the apikey parameter
would fail the length validation check.
Verify: use the VERIFY_URL instead of NOTIFY_URL

The Verify call had a cut-and-paste error, sending verification requests
to the notification endpoint.
add support for developer keys

NotifyMyAndroid supports developer keys that have a higher number of API
requests per hour.  This adds an method to set the developer key for the
NMA object which will then pass it along to the API calls if it is set
(non-empty).
add support for notification priorities

This patch adds constants for defining notification priorities.  The
priority will be passed to the client if it is non-zero ("normal").
add support for URL and Content-Type

NMA now supports URLs in the notification fields, and content-type to
indicate the client should render the description using basic html tags.