Resolves issue #210 (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()
petl.transform.reshape.itermelt() try:except loop now excepts IndexError

This is caused when rows are short - the melt() should return no row if the short row does not have a value for a given field
Merge pull request #275 from rogerkwoodley/issue-210

closes #210