Add Travis and test coverage

Notes:
- npm run test also runs the memory tests, which might be flaky on
  some systems.
- npm run test-coverage only runs the unit tests (it can't run the
  memory test anyway because the memory tests create a Node child
  process, and istanbul cannot cover that).

package.json

@@ -26,14 +26,17 @@
     "requires-port": "1.0.0"
   },
   "devDependencies": {
+    "coveralls": "^2.11.6",
     "eslint": "^2.2.0",
+    "istanbul": "^0.4.2",
     "mocha": "~2.2.4",
     "nock": "~1.9.0",
     "supertest": "~0.15.0"
   },
   "scripts": {
     "lint": "eslint .",
-    "test": "./node_modules/.bin/mocha ./test/test*.js --reporter spec"
+    "test": "mocha ./test/test*.js --reporter spec",
+    "test-coverage": "istanbul cover ./node_modules/.bin/_mocha -- test/test.js --reporter spec"
   },
   "engines": {
     "node": ">=0.6.6",