Add Configuration to NPM for Travis CI Builds

[bcoe]

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:

• 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:

Let me know if you have any feedback 👍, for instance, a strong objection to using Travis CI.

[bcoe]

Ahh, I see that we did have Travis in, but were running into build issues?

a399e65...6aa633e

It seems as though builds are more stable now, with node v0.10+. Worth giving it another shot perhaps?

[bcoe]

This pull should fix the test which is currently being skipped on Travis:

#4663

[bcoe]

I'm closing this pull, based on IRC discussion -- will work to eliminate NPM's external dependencies, then we can move towards CI.