Duplicated modules are now deleted with 'theano-cache clear' as well as automatically once they are old enough

The 'filter' method of a ConfigParam object is not called anymore on the
default value when the ConfigParam is created.

Added a test case to test this new behavior.

Avoid crash in clear_old when the path to the compiledir is not exactly the same as when the module was created

- 'theano-cache clear' will now delete cache entries containing pickled files
that cannot be unpickled.
- Replaced some manual calls to shutil.rmtree by calls to _rmtree to
standardize cache directory deletion.
- Cache entries associated to KeyData objects whose keys are broken should
now be cleared if the module file is gone.
- More comprehensive cleanup of the entry_from_key, module_hash_to_key_data
and loaded_key_pkl dictionaries when deleting a cache entry.

This solves a problem experienced by the buildbot on ceylon.
cuda_ndarray and cutils_ext seem to be the only modules that need
config.compiledir in sys.path to be imported (except when they are
recompiled), so we modify sys.path before trying to import them.

- Temporary work directories created for modules whose compilation process
  does not work properly (e.g. because there is no associated C code) are now
  deleted immediately instead of waiting until the process ends. Note that we
  could probably make it so that we do not create then delete too many
  directories by checking if the C implementation works before creating the
  directory.
- KeyData objects associated to unversioned modules are now created, so that
  the module can be re-used by another unversioned key in the same process.
  They are not saved though.
- Versioned but broken keys can now also re-use modules.
- The version part of unversioned keys is now ignored in the module hash, just
  in case there would be multiple unversioned keys whose string representation
  of the version would be different.
- Added a 'debug_counter' utility function that can be used for debugging
  (may be moved later to a more generic place if someone wants to re-use it
  in another module)