fix minor bug in p3
Add CommonAPI.java
Add ProjectEuler/problem_4.java
Add ProjectEuler/problem_5.java
Add gcd, lsm functions.
Add comment problem links.
Fix minor bug in gcd functions.
Add ProjectEuler/problem_6.java
Add ProjectEuler/problem_7.java
Add ProjectEuler/problem_8.java
Add PythagoreanTriplet.java
Add ProjectEuler/ProjectEuler/problem_10.java
Add ProjectEuler/ProjectEuler/problem_9.java