Removes old commented code
Updates expectations to the latest context API: Now contexts are directly mappings and require no `data` attribute.
Renames EXPRESSION_CONTEXT to EXPRESSION_CAPTURING, which is better named.
Removes some fluff code
Updates history in the docs
Makes _boolean comparable in Py3k but always return _false. TODO: Review.
Fixes bug with backrefs in translation tests.
Fixes ages issues in translation tests.
Removes the default for var in operations. They should fail if needed.
Resurrects the code for all_, any_ and others
Makes all_ and similar accept only sub-queries for the Python translator
Adds a SumFunction to the expression language
Tests the sum function.

TODO: Redesign the translator to clearly separate internal translation values
from vm's values. For instance:: Don't use a lambda as the return of vm
instruction, but an internal callable we can test for without fear of mixing
with user's types.
Updates design tests to new context API
Merge branch 'develop' into translating-py
Implements a basic this_instances protocol in the translation.

If a token has an is_instance filter related and the class in there has a
`this_instances` attribute that is a collection, the translator will use it as
the source of objects.
Reveals a bug in thesefy.

Since we create a class that inherits from the previous super(X, self).method
will recurse instead of traversing upwards.
Fixes bug: thesefy impedes super.

The problem was that thesefy actually modified the class hierarchy, by creating
a returning a new class that inherited from the original one. So super(Target,
self) actually retrieved the hidden original class instead of its base, and all
methods were actually in that hidden original class.

Problem diagram::

        Before thesefy

        +-----------------+
        |  Foobar class   |
        +-----------------+
        |  attrs          |
	|  .....          |
        +-----------------+

        After thesefy

        +------------------------+
        |  hidden Foobar class   |
        +------------------------+
        |  attrs                 |
	|  .....                 |
        +------------------------+
             ^
             |
             |  inherits
        +-----------------+
        |  Foobar class   |
        +-----------------+
        +-----------------+

The solution is to recreate the Foobar from the original's attributes. This is
already what `xoutil.decorator.compat.metaclass` does, we just use it here.
Removes the unneeded Base that was used to workaround a (now fixed) bug in thesefy.
Don't skip tests in PyPy.

Since the introduction of `this_instances` the amount of objects in PyPy is not
relevant for this tests.

Just mark them as xfails cause of the unknown context bug.