checkpoint_dir not found when test_max_length is greater than max_length

For example, assume max_length is 10 and test_max_length is 20:

```
Percent: [####################] 100.00% Finished.
[*] Build a NTM model finished
[*] Reading checkpoints...
Traceback (most recent call last):
File "main.py", line 49, in
tf.app.run()
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/platform/app.py", line 30, in run
sys.exit(main(sys.argv))
File "main.py", line 38, in main
ntm.load(FLAGS.checkpoint_dir, 'copy')
File "/root/NTM-tensorflow/ntm.py", line 257, in load
raise Exception(" [!] Testing, but %s not found" % checkpoint_dir)
Exception: [!] Testing, but checkpoint/copy_20 not found
```

Make a symbolic link from checkpoint/copy_10 to checkpoint/copy_20 works around the problem.

Nice job, thank you!

Start with the argument handling in main.py around lines 38-49, then inspect checkpoint loading in ntm.py around line 257. Reproduce the case with max_length 10 and test_max_length 20, and verify that testing no longer fails because checkpoint/copy_20 is missing.

Written by the indexing model from the issue text.