Moved isSentence() to qutil.py

This change is mostly so we can use it in other tests and convenience scripts,
such as rand_sents.py (see next commit).  It probably belongs here anyway.
Added utility for pulling random sentences from an article

As per Rachel's request, rand_sents.py will pull N random sentences from an
article file and write them to an output file.  Defaults: N = 10, pull from
data/set1/a2.htm, and write to src/test_files/rand_sents.txt.  The comments
at the beginning of the script explain how to easily change the defaults,
either in the file or on the command line.  (Run it in the src/ directory!)