(CFACT-30) Implement SSH facts.

Implementing the following facts on Linux and OSX:

- sshdsakey
- sshrsakey
- sshecdsakey
- sshed25519key
- sshfp_dsa
- sshfp_rsa
- sshfp_ecdsa
- sshfp_ed25519
(maint) Fix Cumulus Linux support to match ruby facter.

Fixing implementation of Cumulus Linux support to match what we do in
ruby facter.
Merge pull request #39 from peterhuene/fix/cumulus

(maint) Fix Cumulus Linux support to match ruby facter.
Merge pull request #38 from peterhuene/facts/ssh

(CFACT-30) Implement SSH facts.
(CFACT-23) Add support for YAML output.

Note: this commit adds a new dependency on yaml-cpp.

Implementing YAML output with the addition of a --yaml (-y) command line
option.
Implementing scalar_value<T> to replace string_value, integer_value,
and boolean_value.