"Keep going" functionality

I discovered that some images are being reported as `application/octet-stream` without `MAGIC_CONTINUE` flag.
`file` utility has `--keep-going` flag which sets `MAGIC_CONTINUE`, I added the same `keep_going` argument to the `Magic` constructor.
I also refactored tests and added `.travis.yml` — so you can integrate Travis CI by just setting up the hook (see repository Settings -> Service Hooks -> Travis for details).