Merge change to phonemetadata.proto from upstream r611
Add carrier metadata from upstream r615
Genericize buildgeocodingdata.py

There is nothing geodata-specific about it, it is easily
adapted to work with arbitrary per-prefix data.
Generate carrierdata Python files
Include carrierdata directory in packaging
Make area_description_for_number generic.

It looks up a number in a prefix-indexed set of
data, and is not geocoding-data specific.
Don't export geocoder functions at top-level.

Require these to be explicitly imported via
phonenumbers.geocoder, to allow for similarly-named
functions in incoming phonenumbers.carrier submodule.
Implement carrier mapping, analogous to upstream r615