Haskell: Add solutions for problems 1-20.
Clojure: Fix problem 1.
Clojure: Fix prime-factors, move it to util.

prime-factors now takes into account for powers of individual primes.
Clojure: Speed up n-divisors, move it to util.

n-divisors now uses the prime factorization of its input.
Clojure: Speed up problem 4.
Clojure: Fix whitespace issues.
Clojure: Speed up digits-of.
Clojure: Move fibonacci-sequence to util, fix typo
Clojure: Move is-palindrome? to util, drop 'is-'.
Clojure: Add opt. base arg. to digits-of.
Clojure: Add fn for checking if a number is prime.
Clojure: Add proper divisor sum function.
Clojure: Make solutions functions.

This commit formulates all solutions as parametrised functions instead
of simple statements.
Clojure: Add max-index function.
Add data folder for problem files.
Clojure: Add solutions for problems 21-50.
Haskell: Make solutions functions.
Haskell: Add solutions for problems 21-30.
Haskell: Add a few imports to util.
Haskell: Make run print results, not return them.