Update Instance Spec for api_version

Update the Instance spec to include tests for api_version (as well as
the Factory).
Update README and Tomcat::Manager for api_version

Update the README and Tomcat::Manager new method documentation to
include examples of specifying the extra option of api_version.
Update Instance for Specifying api_version

Update the Instance class to include being able to specify (and
gracefully fail if incorrect) a specific API version.