Add Configuration to NPM for Travis CI Builds

I've added configuration that allows NPM to be built with Travis CI. Mainly, this required the addition of a .travis.yml file to the root directory of the repo.

A few notes:
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* **test/tap/lifecycle-signal.js** fails on Travis, but runs locally. I could reproduce the problem locally by running: `sudo node -e 'process.kill(process.pid,\"SIGSEGV\")'` as a preinstall step, rather than `node -e 'process.kill(process.pid,\"SIGSEGV\")'`. Makes me think it has something to do with the permissions with which a travis agent executes a child-process. I could not get to the bottom of the problem, and have disabled this test for Travis (for the time being).
* **test/tap/publish-config.js** hangs in node v0.8.26, I can reproduce this locally. For the time being, I've configured travis to run with a 0.11 agent, and a 0.10 agent.
* You can see builds passing here: https://travis-ci.org/bcoe/npm
* I've added a badge to the README, that indicates build status:

![screen shot 2014-03-17 at 1 22 57 pm](https://f.cloud.github.com/assets/194609/2441264/f7d3847e-ae11-11e3-8277-ab6ede6d3ba0.png)

Let me know if you have any feedback :thumbsup:, for instance, a strong objection to using Travis CI.