Add .path_to_docgen to SDK class

- refactor
- add sdk.SDK.path_to_docgen property
Add ':' after Dart in build system names
Add basic docgen tasks to build system

- Build: Dart: Generate Documentation (from project dir)
- Build: Dart: Serve Documentation (from project dir)
Merge pull request #174 from my-personal-forks/docgen

Basic support for docgen