The fix mostly includes updates to modutils, which is
modified in order to properly lookup paths from live objects,
which ends in $py.class, not pyc as for Python 2,
Closes issue #83.

There were two problems until now:

  * first, the transformed builtin object was deleting during clear_cache,
    leaving the next tests to use the untransformed tests, which yielded
    InferenceErrors.

  * after the first problem was fixed, another problem with CONST_PROXY
    occurred, since it was creating a new builtins object, without
    the builtins transformations applied, which was used for creating
    the const classes. The problem was that even replacing the
    builtins instance in astroid_cache, InferenceErrors occurred,
    because the const classes were already built and they were referencing
    the untransformed builtins. That's why _astroid_bootstrapping
    allows to receive a new builtins object, which is passed
    properly by the tests.

Also modernized a lot of test code. This makes it possible to run the
tests from both the base directory of the project as well as the
astroid package.

Since the packages have the same name for Python 2 and 3, there is less
version-dependent code in the tests.