Added new module testing.tools.

Added
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* New module landlab.testing.tools contains simple wrappers for
  assertions not available to Python 2.6.
Added testing for Python 2.6.
Changed to fallback to landlab.testing assertions.
Changed to use nose rather than unittest.
Changed assert_raises to use old-style call.
Added assert_list_equal, assert_tuple_equal.
Changed to use nose instead of unittest.
Added import of assert_equal from nose.
Added assert_set_equal, assert_dict_equal.
Changed to use nose rather than unittest.
Changed to use nose rather than unittest.
Changed to use nose rather than unittest.
Changed to use nose rather than unittest.
Changed to use nose rather than unittest.
Changed to use nose rather than unittest.
Changed assert_raises to use old-style call.
Merge pull request #38 from mcflugen/add-2.6-support

Add Python 2.6 support