Resolves issue #209

Resolves issue #209 (https://github.com/alimanfoo/petl/issues/209): melt() chokes on non-string key arg

petl.transform.reshape.itermelt() only requires that key is iterable

Existing code accepted key as either a basestring or as an iterable object.  ints, floats, etc. failed these cases, and resulted in non-iterable keys

unittest added: petl.test.transform.text_reshape.test_melt_non_string_key()