Name the app :dotenv, not :dotenv_elixir

The name of the repo is sufficient to identify it as an Elixir project,
but you should be able to use it as :dotenv in your mix.exs
Update elixir dependency to accept 0.15.0
Merge pull request #6 from lsimoneau/elixir-0.15

Update elixir dependency to 0.15.0
Merge pull request #5 from lsimoneau/rename-app

Name the app :dotenv, not :dotenv_elixir
Document public functions, add typespec, make private functions private