Fstringen

Fixing an issue in the documentation and the example.

The API should stabilize soon and a v1 release is to be expected.

A bit more of API breakage (this is still v0.0.*):
- `Model.from*` methods have been removed; please use `Model(name, value)` instead.
- Fixed a bug that made `Model.select("path", None)` return None if the path is not found.

The latest release was quite solid, and this one is just improving things. The API should stabilize soon and a v1 release is to be expected.

Significant API breakage (this is still v0.0.*):
- `gen` no longer takes the `comment` and `notice` args, use `preamble` instead;
- `root` arg is no longer exposed by the `Model.from*` static methods (it didn't make sense);
- `callerctx` arg is no longer present in `Model` methods, as it was no longer in use;
- Streamlined docstrings;
- Code was moved around and better organized in different files;
- README improvements.

Significant API breakage (this is still `v0.0.*`):
- `Selectable` was consolidated into `Model`;
- `mymodel(...)` as a shorcut for `mymodel.select(...)`;
- Default values can be passed to `Model.select` in case a lookup fails;
- `Mapper` was removed;
- The undocumented lookup local vars for path parameters (e.g., `/path/<myvar>/name`) was removed;
- Improve reference lookup in `Model.select` so that it's more consistent (`/path/object/name->` and `/path/object/myref->` work the same way now);
- Bug fixes, README improvements.