Make composer.json optional as discussed in issue #840.

The patch includes the following changes:
* Update theme.xsd to allow version
* Update theme.xsd to allow parent
* Update Config\Theme to parse version and parent from theme XML config
* Update Config\Theme to merge in composer.json config data
* Add validation to check parent and version match if specified in both files
* Add validation the title is present in XML theme config
* Add unit tests for changes

Please refer to the issue https://github.com/magento/magento2/issues/840 for further details.