(CFACT-41) Use resolv.conf for domain name fact resolution.

If we can't resolve the hostname, look for domain and search entires in
/etc/resolv.conf.

This allow us to conform to what facter is doing even when the hostname
is not resolvable.
(CFACT-42) Normalize external fact names to lowercase.

When processing JSON, YAML, text, and executable external facts,
normalize the fact names to lowercase to match how facter handles
external fact names.
(maint) Rename fact_resolver to simply "resolver".

The name "fact_resolver" is a bit redundant and we already use
"resolver" in the external namespace for external fact resolvers.
Therefore, renaming "fact_resolver" to simply "resolver".
(maint) Rename "fact_map" to "collection".

Renaming "facter::facts::fact_map" to "facter::facts::collection".

The name "fact_map" is a little redundant (it's already in the "facts"
namespace).
(maint) Replace write_json, write_yaml, and operator<< with write.

Replacing write_json, write_yaml, and operator<< with a single write method
that takes the format to use as an option.
(maint) Fix potential fact resolution cycle in OS fact resolver.

The POSIX OS fact resolver references the operatingsystem fact while
resolving the osfamily fact.  However, it does not pass false while
retreiving the fact, causing a cycle if the kernel fact wasn't present
for some reason.
(maint) Add a can_resolve member to external fact resolvers.

Currently the resolve member of external fact resolvers returns false if
the resolver isn't responsible for files of the given type.

This logic belongs in a "can_resolve" member that can be used to check
if the resolver can resolve facts in the given file.

This allows the fact collection to be able to map an external facts file
to the responsible resolver.
(CFACT-40) Refactor fact evaluation for future custom fact support.

Refactoring facterlib for the coming support of custom facts:

* Adding --custom-dir and --no-custom-facts command line optinons (not yet implemented).
* Refactoring fact evaluation so that custom facts can add resolvers via a Ruby API.
* Resolvers can now overlap on the facts they resolve; the last resolver that was added to the collection will be the one that resolves the fact.
(maint) Update README example for API changes.

Updating the example in the README for recent API changes.