add LcsComparator which calculates similarity of two strings. Close #3

Implement LCS algorithm and add ```LcsComparator``` class.
But current code only can calculate the length of LCS between 2 strings.
Moreover, score(similarity) is calculated simply by dividing length of LCS by length of reference string.
So if reference string is ```"abcd"``` and comparison string is something like ```"a.....b........c......d"```, score is calculated as 1.0.