How do I load an Elixir library into iex without adding it to a project's mix.exs deps?

I want to tryout the Poison json module without creating a mix project.

How do I install it and make it available in iex via import?

I have been able to add it to a project, then use it after going into the project directory and using iex -S mix:

tbrowne@LILJEN:~/code/elixirTry/pj$ cat mix.exs
defmodule Pj.Mixfile do
  use Mix.Project

  def project do
    [app: :pj,
     version: "0.0.1",
     elixir: "~> 1.2",
     build_embedded: Mix.env == :prod,
     start_permanent: Mix.env == :prod,
     deps: deps]
  end

  # Configuration for the OTP application
  #
  # Type "mix help compile.app" for more information
  def application do
    [applications: [:logger]]
  end

  # Dependencies can be Hex packages:
  #
  #   {:mydep, "~> 0.3.0"}
  #
  # Or git/path repositories:
  #
  #   {:mydep, git: "https://github.com/elixir-lang/mydep.git", tag: "0.1.0"}
  #
  # Type "mix help deps" for more examples and options
  defp deps do
    [{:poison, "~> 2.0"}]
  end
end
tbrowne@LILJEN:~/code/elixirTry/pj$ cat lib/pj.ex
defmodule Person do
  @derive [Poison.Encoder]
  defstruct [:name, :age]
end

defmodule Pj do
  xx = Poison.encode!(%Person{name: "Devin Torres", age: 27})
end

tbrowne@LILJEN:~/code/elixirTry/pj$ iex -S mix
Erlang/OTP 18 [erts-7.2] [source-e6dd627] [64-bit] [smp:2:2] [async-threads:10] [hipe] [kernel-poll:false]

Interactive Elixir (1.2.3) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> import Poison
nil
iex(2)>

However if I just go into a normal iex in a generic directory then I can't seem to access the Poison library:

iex(4)> import IO
nil
iex(5)> puts("hello")
hello
:ok
iex(6)> import Poison
** (CompileError) iex:6: module Poison is not loaded and could not be found

Also, how do I install a library globally from github?