Resolves issue #176

Passed Travis tests

Resolves issue #176 (https://github.com/alimanfoo/petl/issues/176): addfield() and uneven row lengths

The addfield() function now pads short rows with None to ensure proper alignment.

unittest added: petl.test.transform.test_basics.test_addfield_shortrow()
Sorry, could you explain why try:except is needed?