This calculates the similarity between two strings as
described in Oliver [1993]. Note that this implementation
does not use a stack as in Oliver's pseudo code, but
recursive calls which may or may not speed up the whole
process. Note also that the complexity of this algorithm is
O(N**3) where N is the length of the longest string.
By passing a reference as third argument, similar_text() will calculate the similarity
in percent for you. It returns the number of matching chars
in both strings.