Add longestPalindrome.cpp

Accepted.
O(N^2) complexity, a O(N) algorithm can be found
at leetcode.com/2011/11/longest-palindromic-substring-part-ii.html

Signed-off-by: Wayne Wang <wythern@gmail.com>
Add common.h for common header files and functions.

Signed-off-by: Wayne Wang <wythern@gmail.com>