Fix loopback in PhantomJS, fix karma tests

- Move configuration of Karma unit-tests from `Gruntfile.js` to a
  standalone file (`test/karma.conf.js`).

- Add a new Grunt task `karma:unit-ci` to run Karma unit-tests in
  PhantomJS and produce karma-xunit.xml file that can be consumed
  by the CI server.

- Add grunt-mocha-test, configure it to run unit-tests.

- Add `grunt test` task that runs both karma and mocha tests,
  detects Jenkins to produce XML output on CI server.

- Modify the `test` script in `package.json` to run
  `grunt mocha-and-karma` (an alias for `grunt test`).
  The alias is required to trick `sl-ci-run` to run `npm test`
  instead of calling directly `mocha`.

- Add `es5-shim` module to karma unit-tests in order to provide
  ES5-methods required by LoopBack.

- Fix `mixin(source)` in lib/loopback.js to work in PhantomJS.
  `Object.getOwnPropertyDescriptor()` provided by `es5-shim` does not
  work in the same way as in Node.
package: upgrade juggler to 2.0.0-beta2