Initial commit.

Added an improved version of calculating distance between two text files.
The improvement is achieved by reducing one “for” loop over one of the
word_freq_dict, because when you calculate the inner product of
word_freq_dict1 and word_freq_dict2, you can reuse the same loop to
calculate the inner product of word_freq_dict1 itself.
For more information, please refer:
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-006-introduction-to-algorithms-fall-2011/