Add Travis build to project

Travis is a free (for open source projects) continuous integration server, whereby a build will be triggered and all tests run on every push to the project. The status of that build, if the tests pass or fail will be indicated by comment being made on any pull request or via the included image in the repo README.

Just merging this pull is not however the be-all-and-end-all, there needs to be a small amount of config done at https://travis-ci.org in order for it to built there for this core repo (and not just my fork, as it is now).

1. Go to https://travis-ci.org, click 'Sign in with Github'. Sign in.
2. Hover over your account name in the top right, click 'Accounts' from the ensuing dropdown.
3. A list of your repositories will show up, if it doesn't click 'Sync now' and wait for them to show up.
4. Find 'natural' on the list. Make sure the toggle is on. That's it.
5. Push to the repo, and it will trigger a build on travis-ci.

This will build for every branch on the project. This should give a much better idea as to the status of branches and their test integrity, do they break any tests etc.