ignore IDEA control files
Solved P11
reviewed isPalindrome. It now reuses reverse. I actually copied this solution from other users.
solved P12
Fixed last() and penultimate() to support empty list or shourt lists ;-)
Solved P13
Solved P14
Added comment on a possible improvement
solved P15. Also removed usage of flatten, replaced with flatMap(identity)
Solved P16
Solved P17
Merge pull request #12 from ignasi35/master

Solved P11 to P17