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.