(CFACT-138) Implement Google Compute Engine fact.

Implementing the gce fact that returns instance metadata for Google
Compute Engine virtual machines.

The structure of the fact's data is dependent upon the JSON being
returned from Google's metadata endpoint.
(maint) Command line queries should be case-sensitive.

Previously we were lowercasing each query from the cfacter command line.
This prevented us from being able to query map elements that are not all
lowercased.

cfacter should simply be case-sensative from the command line, but
remain case insensitive via the Facter API.
(maint) Fix some missing doc comments.

Fixing some missing doc comments on to_utf8 and to_utf16 utility
functions.