Gen: a new transform.Transform

[jonasmalacofilho]

This has several objectives...

Syntax/transform:

• [x] improved behavior of chapter counter: globally increasing, but zero just after \volume and before \chapter (support for volume and document landing pages)
• [x] cleaner tree and implementation
• [x] null-safe resulting tree
• [ ] move empty element checks (i.e. section names) from the parser to the transform
• [ ] support for manually setting id (and better support for automatic ids)
• [x] fix #1
• [ ] more tests
• [ ] documentation for tree and implementation

TeX/PDF generation:

• [x] tex label generation

Html/website generation:

• [x] new html navigation

[jonasmalacofilho]

Continues on #6