Added idea files to gitignore.
Added problem #1 solution.
Added solution for second problem.
Added sieve of eratothenes reailizations: custom and rosetta code.
Solved problem #3.
Solved problem #4. Added method palinrome? to fixnum.
Solved problem #5.
Solved problem #6.
Solved problem #7.
Solved problem #8.
Solved problem #9.
Solved problem #10.