Merge pull request #31 from MSch/master

Add Plug.Adapters.Cowboy.child_spec/3 so you can supervise your Plugs.
Add tests for cowboy adapter
Move Plug.Conn to a struct
Rename Plug.Connection to Plug.Conn
Remove space after { and before }
Move Plug.Upload to a struct
Update .travis.yml file
Convert more records to structs
Remaining record and upgrade comprehensions
Convert keywords in params, cookies and assigns to maps
Add get_resp_header and get_req_header
Release v0.4.0 with v0.13.0 support
Bump exdoc dependency
Fix deprecation warnings in 0.13.1
Properly document conn/3 for tests
Add adapters.ex to check available adapters
Port mime types to Plug
Release v0.4.1
Add a description, bump to 0.4.2-dev
Move adapters to check to adapter file to avoid recompilation