(maint) Add unit tests for util and execution.

Adding tests for:

- facter::util::file
- facter::util::option_set
- facter::util::posix::scoped_descriptor
- facter::util::posix::scoped_addrinfo
- facter::util::bsd::scoped_ifaddrs
- facter::execution::posix
(maint) Add test target to cmake output.

Adding a test target to cmake output so tests can be run with just "make test".
Updating travis to run tests through ctest.
Updating README to cover running tests.
(maint) Add unit tests for logging.

Adding unit tests for debug, info, warning, error, and fatal logging.
(maint) Add tests for array_value.

Adding unit tests for facter::facts::array_value.
(maint) Add tests for string_value.

Adding tests for facter::facts::string_value.
(maint) Add tests for integer_value.

Add additional unit tests for facter::facts::integer_value.
(maint) Add tests for map_value.

Adding unit tests for facter::facts::map_value.