Add configuration for Travis CI

[ErichDonGubler]

This is my strong opinion and I hope it doesn't come across as an imposition: CI like this would have helped you realize that things were broken on Linux before cutting a 0.3 release! This is definitely YOUR crate, though, and it's your call whether or not this is appropriate for the development you do here. That's not a wrong decision either!

This Travis config is borrowed from the great-ci repo. Some functionalities you may not actually want even if you decide you want to accept this form of CI generally might be:

• Code coverage analysis
• Putting docs into a gh-pages branch

...which can easily be removed, no worries! 😄

This PR has a soft dependency on PR #9 -- N.B that CI will break without that pass. :)

[amodm]

Thanks @ErichDonGubler. Much appreciated. My dev env is OSX, so this is particularly helpful.