Add ability to halt plug stacks
Remove halt_reason
Remove Wrapper plugs
Remove need for Code.ensure_compiled? and bump docs
Run tests on otp 17.0 and 17.1 on travis

Signed-off-by: chatgris <julien@stormz.me>
Merge pull request #91 from chatgris/travis_otp_versions

Run tests on otp 17.0 and 17.1 on travis
Merge pull request #90 from chrismccord/cm/halt-plug-stacks

Add ability to halt plug stacks