Large refactoring of JsDuck::App class.

Splitting the output routines into three classes:

- StdoutExporter
- JsonExporter
- HtmlExporter

Additionally creating ClassWriter class for outputting the .js or .json
files. This one isn't pretty, but I have to share the output logic
between JsonExporter and StdoutExporter.

Another nastiness is the format_classes method which has to run BEFORE
writing out classes JSON, but AFTER the source HTML files have been
created - because of the parallel stuff. Brr...

At least the App class now has less responsibilities, but further
refactorings are needed.