Remove unused grunt tasks & deps, small README fix

Various fixes in this pull request, all minor.

It took me a while to realise that, although grunt test is defined, it's not working and probably will never be.  I have removed it, along with associated dependencies.  grunt-contrib-jasmine uses PhantomJS, which cannot possibly test a Chrome extension.  Further, I'm not sure Selenium would be able to either.

Also a small README fix to say use src/ when loading unpacked extension.

Current tests continue to pass - though I only see 2 on test.html.  It looks like there are far more in src/js/test/spec/*.

Hoping I haven't misunderstood something about the grunt side of the Jasmine setup.