Async engine shutdown on SIGINT and SIGTERM
This is mostly infrastructure release. The only functional change is shutdown of async engine on SIGINT and SIGTERM signals.
Infrastructure changes:
- Setup of Travis continuous integration
- Most of use cases are now covered (tests run automatically by COVERALLS)
- Switched to OCaml 4.02.3 compiler and new async library
- Added libffi library to Mac OS X binary distribution