Make YAML optional

validate composer
Add composer installation instructions to each package. Closes #13
Kickstart a travis build
Tweak usage of Symfony YAML
Merge pull request #166 from dongilbert/installation

Add composer installation instructions to each package. Closes #13
Merge branch 'yaml-tweak' of https://github.com/fabpot/joomla-framework into JRegistryFormatYaml
Clean up code style. Update tests.
Merge pull request #165 from dongilbert/JRegistryFormatYaml

Make YAML optional