[elixir] Add nucleotide-count exercise
[elixir] Implement count of single nucleotide
[elixir] Implement nucleotide-count w/ all passing tests

This solution does not have any interaction between DNA.count/2 and
DNA.nucleotide_counts/1, which probably defeats the point of the
exercise. I'll be looking at a refactoring.

Also, [DocTests are awesome][1].

[1]: http://elixir-lang.org/docs/master/ExUnit.DocTest.html
[elixir] Implement nucleotide-count w/ DNA.count

Submission: http://exercism.io/submissions/faae51202879a7a4b3f2be66