Add Longest Palindrome Subsequence

[aamodpant15]

Add LPS.

For example, s1 = 'ACBDACDBA',
    LPS(s1) ='ABDCDBA'.

To see this clearly, we can align and capitalize the letters 
that are chosen to be a part of the LPS (ignore the spaces):

s1 =      'A c B D a C D B A',
           |   | |   | | | |
LPS(s1) = 'A   B D   C D B A'.