Added assigning a row from another row. See issue #770

Added a type check for Row.  Also added a test in test_table.py for row-to-row assignment.  The one test failure I'm seeing also fails on a clean build from master, so I think it's unrelated:

    NameError: global name 'download_file' is not defined

    1 failed, 3889 passed, 111 skipped, 3 xfailed in 239.51 seconds

One item I was unsure of: assigning a row containing elements with different data types is not successful.  It doesn't raise an error, but it doesn't successfully assign the new value either.  Not familiar enough with data types to say if that matters or is expected cross-data-type.