Change to YAML syntax definition

There are a few minor changes in this pull request but the main one is a change to using a YAML file for syntax definition. This has resulted in quite large diffs even though the underlying regular expressions have not changed due to trivial formatting differences.

The YAML->PLIST converter I am using is the new SerializedDataConverter package found here:

https://github.com/facelessuser/SerializedDataConverter

I think YAML is much easier to read and edit than XML and it makes sense to make changes to it and let the machines deal with the raw .tmLanguage file.