Change Jinja2 dependency to >= 2.4 (was == 2.7).

Yaml config, support for unexpanded_templates, support non-HTML text-…

…based templates.

Implement complexity.json. Get rid of output_dir arg.

Rename json/ to context/ and json_dir to context_dir.

* Support for multi-level template directories.

* Skip non-HTML files in `templates/` rather than raising `NonHTMLFileException`.

* Improved static site generation API - better parameters are used.

* Files in the root of `assets/` (or the asset directory) now get copied over to the output.
* Much more documentation.

Make reading of JSON files from `json/` optional.

Fix reading of JSON files from `json/`.

Project layout and assets follow a standard structure.

* Project layout is now::

    my_repo/
    ├── project/       <--------- input
    │   ├── assets/
    │   │   ├── css/
    │   │   ├── js/
    │   │   └── img/
    │   └── templates/
    │       ├── base.html
    │       ├── index.html
    │       └── about.html
    └── www/          <---------- output (generated)
        ├── index.html
        ├── about/
        │   └── index.html
        ├── css/
        ├── js/
        └── img/

* Assets are copied over to `www/` during site generation.
* If the `www/` directory was previously created, it prompts the user and then
  deletes it before site regeneration.
* Templates starting with `base` are not generated as individual pages. They
  are meant to be extended in other templates.

* Graceful shutdown/restart of dev server.

* Required input and output dir arguments.
* Optional port argument.
* Improved server start/stop messages.
* Major internal refactor.