composer.json: added a bunch of new settings.
Rationalized the role of the shared interface.

Now both Polyglot and the Context instances share a Translatable interface.
Heavily reworked the Context class internals.

- Removed the `path' and `encoding' members: useless; the gettext binding is
  now managed at register() level.

- Made the `interpolate' function public, as per the Translatable interface.

- Removed some testing & debugging code.
Heavily reworked the Polyglot class.

- Added the `path' and `encoding' members: now the gettext context binding
  is managed at register() time.

- Made the Polyglot class a singleton.

- Added a magic `__call' method to setup the locale automagically.

- The constructor now checks whether the gettext extension is available.

- The constructor now provides a default locale to work with.

- The `get' method has been renamed to `getContext'.

- The `registerContexts' method has been renamed to `register'.

- The `register' method now binds the gettext context to path & encoding.

- The `registerContext' method has been deleted.

- The `interpolate' and `translate' methods now do a sanity check.

- PhpDoc'ed the whole thing. Yay!