ENH: Add generic get_data_path function.

CPython docs warn that inspect.currentframe "relies on Python stack
frame support in the interpreter, which isn’t guaranteed to exist in all
implementations of Python." However, I tested this in PyPy too and it
works. Yeah!
STY: pep8
TST: Replace get_data_path
added tests of k_words
added initial k_words implementation
added documentation
some code clean-up
added k_word_counts and tests - will be useful for developing alignment-free distance functions
fixed typo
added test of k_word_frequencies
added k_word_frequencies - this will allow some simplification of code in the SequenceCollection objects
addressed comments from @wasade
fixed typo
fixed typo; modified sequence_frequencies to call new method of Sequence object, and moved to SequenceCollection from Alignment
Merge branch 'issue-25' of github.com:gregcaporaso/bipy into issue-25
modified based on comment from @wasade
bug fix
renamed method for clarity
fixed typo
Merge pull request #264 from gregcaporaso/issue-25

add k_words function to the BiologicalSequence object