Fix slice and __getitem__ methods of padics

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/13299">trac #13299</a>.

fixed new-style slices

Reported by: saraedum

Component: padics

CC: roed

Report Upstream: N/A

Authors: Julian Rueth

Dependencies: 

Work issues: 

Reviewers: Rob Harron, David Roe

Merged in: sage-5.9.beta1

Attachments: <ol><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/13299/trac_13299.patch">trac_13299.patch: </a> by saraedum at 20130213T16:58:29</li><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/13299/13299_review.patch">13299_review.patch: </a> by roed at 20130316T19:41:49</li><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/13299/trac_13299_review2.patch">trac_13299_review2.patch: fixed new-style slices</a> by saraedum at 20130318T06:16:42</li></ol>