Fixing package dependencies

autobahn and Twisted are strictly vtkweb plugin dependencies, not Tangelo
dependencies
Converted tangelo config processing to yaml
Service config files now use yaml

Still works with JSON configs in, e.g., flickr demo
Converted other JSON-using config paths to yml files
Removing unneeded minify_json, as well as superfluous import statements
Updating docs to reflect YAML config files
Converted plugin configuration to yaml
Converted all ".yml" config files to ".yaml" following official suggestion of the yaml people
Merge branch 'yaml' into plugin-system

Closes #417