grunt: move Travis ESLint configuration to Grunt (#21)

Install the grunt-cli somewhere global on your system. This gives you
the "grunt" command, which will use the version of Grunt installed
locally for your project.

Then run "grunt eslint" to start eslint with the same options as we
use on Travis.

The explicit dependency on eslint is no longer needed -- the
grunt-eslint module has its own dependency on eslint.
karma: update to version 0.12

We will need this version when we begin using grunt-karma in a
subsequent commit. Unlike karma 0.10, version 0.12 no longer depends
on a number of plugins:

  https://github.com/karma-runner/karma/commit/e033d5618a98

We now explicitly depend on the plugins we use.
grunt: add karma tasks (#21)

This adds karma:{unit,single,coverage,travis} tasks. This will let us
consolidate the configuration more in the future.
package: remove obsolete scripts

There are now equivalent and shorter grunt tasks for these scripts.
protractor: upgrade to 1.2

This is the version grunt-protractor-runner uses.
grunt: add protractor task (#21)

The grunt-protractor-runner task depends on protractor, so we no
longer need to depend on it explicitly.

The latest version of grunt-protractor-runner is 1.1.4 at the time of
writing. That version moved the installation of the webdriver from a
postinstall script to a pretest script. Without the

  webdriver-manager update

call, one cannot run protractor. With grunt-protractor-runner, the
webdriver-manager script resides in

  node_modules/grunt-protractor-runner/node_modules/.bin/webdriver-manager update

which is a quite unwieldy path. So instead of referencing it from our
package.json file, I opted to use grunt-protractor-runner 1.1.3 which
knows how to run webdriver-manager automatically.

The update-webdriver and protractor npm scripts are now obsolete, the
latter has been replaced by 'grunt protractor'.
grunt: add coveralls task (fixes #21)
travis: remove now obsolete $PATH setting

All tasks are now executed using grunt and we don't need to invoke
scripts from node_modules/.bin any longer.