feat(lib): switch to non-blocking (asynchronous) IO

BREAKING CHANGE: This breaks a lot of the Client and Server APIs.
  Check the documentation for how Handlers can be used for asynchronous
  events.

.travis.yml

@@ -3,6 +3,8 @@ matrix:
     fast_finish: true
     include:
         - os: osx
+          rust: stable
+          env: FEATURES="--no-default-features --features security-framework"
         - rust: nightly
           env: FEATURES="--features nightly"
         - rust: beta