Implement list.clear method

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-227 into branch/py3.3

Created originally on Bitbucket by kgriffs (Kurt Griffiths)

Was already merged in Bitbucket before import, marked as merged by the import user

This patch simply adds descr_clear to W_ListObject and has it call the internal clear() method.

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