Copter: Issue 901 severity values

partial fix for #901 . 

Here's how I changed severities:

was|is
----------|----------
SEVERITY_LOW | MAV_SEVERITY_WARNING
SEVERITY_MEDIUM | MAV_SEVERITY_ALERT
SEVERITY_HIGH | MAV_SEVERITY_CRITICAL
SEVERITY_USER_RESPONSE | MAV_SEVERITY_NOTICE

I made the mavlink library require an `int` rather than a `gcs_severity` because copter is now using `MAV_SEVERITY`s but all else is using the incorrect `gcs_severity`.  Once the other parts of the project have been fixed, then the library can be changed to accept a `MAV_SEVERITY` and the old enum `gcs_severity` can be removed.