feat: json_library set to Jason by default (#182)

Also:
- Added initial Contributing section to readme
- Version set to 2.0.0-dev

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## v2.0.0 (dev)
+
+**Changed**
+
+- Default JSON library set to Jason.
+
 ## v1.6.0
 
 **Added**

README.md

@@ -275,3 +275,18 @@ more complex.
      end
    end
    ```
+
+## Contributing
+
+### Testing
+
+Unit tests can be run with `mix test` or `mix coveralls.html`.
+
+### Formatting
+
+This project uses Elixir's `mix format` for formatting. Add a hook in your editor of choice to
+run it after a save. Be sure it respects this project's `.formatter.exs`.
+
+### Commits
+
+Git commit subjects use the [Karma style](http://karma-runner.github.io/5.0/dev/git-commit-msg.html).

lib/pigeon.ex

@@ -95,7 +95,7 @@ defmodule Pigeon do
   """
   @spec json_library :: module
   def json_library do
-    Application.get_env(:pigeon, :json_library, Poison)
+    Application.get_env(:pigeon, :json_library, Jason)
   end
 
   @doc false

mix.exs

@@ -1,7 +1,7 @@
 defmodule Pigeon.Mixfile do
   use Mix.Project
 
-  @version "1.6.0"
+  @version "2.0.0-dev"
 
   def project do
     [
@@ -57,9 +57,9 @@ defmodule Pigeon.Mixfile do
       {:ex_doc, "~> 0.18", only: :dev},
       {:gen_stage, "~> 0.12 or ~> 1.0"},
       {:httpoison, "~> 0.7 or ~> 1.0"},
+      {:jason, "~> 1.0", optional: true},
       {:joken, "~> 2.1"},
-      {:kadabra, "~> 0.4.3", optional: true},
-      {:poison, "~> 2.0 or ~> 3.0 or ~> 4.0"}
+      {:kadabra, "~> 0.4.3", optional: true}
     ]
   end