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.
With current dependencies (especially with nekohtml 1.9.21), these statements causes an exception and test fails.
But on inquiry, upper statement (line 38) does not cause problem.
I will uncomment this statement in next commit.
What do you think about this?