Add continuous integration build testing

This PR adds support for Travis CI automated build/commit testing.

With this, all commits/PRs to the sstur/nodeftpd repo will be automatically built by https://travis-ci.org/ and have the test suite run.

The first commit instructs Travis to test against each of the two most recent Node versions.

The second commit adds an image to the README file which indicates the build status of master (passing/failing/unknown).
see https://github.com/asylumfunk/nodeftpd/tree/i/45

Note:
- For this to function, the Github webhook will need to be activated.
- @sstur will need to do this (as repo owner) by following steps 1 and 2 at http://docs.travis-ci.com/user/getting-started/ .
- Steps 3 and 4 are handled implicitly by merging in this PR (to be merged AFTER steps 1 and 2).

@sstur, you'll want to merge this PR immediately after activating the webhook. If any other commits are pushed (either directly committed or a PR submitted) after the webhook is activated, but before the `.travis.yml` file exists in the repo, the build will always fail. This happens because Travis assumes that projects without a `.travis.yml` file are Ruby projects and runs tests accordingly.

Hopefully, I didn't manage to make the process sound any more difficult than necessary, so of course, let me know if you have any questions.