Equivalence to the double metaphone primary value.

This commit establishes equivalence to the Apache commons
implementation of this algorithm for the primary value. Using a
corpus of 138K words from the "aspell" program, I generated test
cases using the reference (java) implementation. I then put these
inputs through this (golang) implementation to see if their values
were equal. Comparisons using the secondary/alternate values
between the two will be next.