updated methods and tester

The program was throwing an exception for me after test 3. Catenation,
GetElementAtIndex, addLast, and addAll now seem to be working correctly.
I also tested addFront to make sure things were working and it passed my
test. There was also an indexing problem in test 6 that I believe is now
fixed.

I went ahead and reused the original DoubleLinkedSeqTester to make sure
I was using the right version. The program can get all the way through
the last test now.

I'll just attach my current build to this email. If you guys want me to
submit it to Google drive, or GitHub let me know. Also, let me know what
you think of the build.