Resolves issue #210

Passed Travis test

(https://github.com/alimanfoo/petl/issues/210): melt() chokes on short rows

Added a try:except to pet.transform.reshape.itermelt()

melt() now returns no (key, variable, value) row when the value is missing for a given row

unittest added: petl.test.transform.text_reshape.test_melt_1_shortrow()
petl.test.transform.text_reshape.test_melt_2_shortrow()
Small point, but maybe should catch IndexError here.