RestText format.
Add a FIXME for an unfinished feature.
Separating data for context levels.
Merge de docs con manu.
Modificaciones parciales de los contextos.
Finishes the first version of xoutil.collections.StackedDict
Fixes :func:`xoutil.decorator.compat.metaclass` so that slots work
Fixes OpenDictMixin to take account of slots in all the hierarchy
Fixes StackedDict API
Updates contexts to be stacked dicts. Removes the `data` attribute from
contexts.
Fixes opendict implementation to avoid methods shadowing.

The previous implementation allowed to hide dict's method like get,
__getitem__, etc..

Since __getattr__ is only called if such an attribute does not exists, it makes
sence a simple implementation like the one this commit introduces.

Alternatively one may set __dict__ to self, but that requires a sure round to
__getattribute__ to avoid the methods shadowing. So, the suppossed efficiency
gained in direct access to __self__ is lost by calling __getattribute__.
Fixes OpenDictMixin and StackedDict docstrings.
Fixes the opendict docstring.
Merge branch 'med-develop' into develop
Explitly raises a RuntimeError if the context is reused directly.
Improves the changeslog in the documentation
Implements the get method for NullContext
Skips tests of xoutil.string