Port run_tests.sh to grunt

2 new grunt tasks. Removes ``tools/run_tests.sh``.

```
# writes a build.prop file for jenkins
# with branch, sha and build num and save to jenkins vars
grunt build-prop

# runs all tests in the shell, requires shell to be stage
# at minimum 'grunt build stage' or simply 'grunt full-build' in brackets-shell
grunt test-integration
```

``test-integration`` task has optional cmdline args:

* ``--shell-repo=<path>`` specify non-default path to brackets-shell repo
* ``--shell=<path to executable>`` specify non-default path to brackets binary (overrides shell-repo)
* ``--spec=<spec name>`` default ``all`` 
* ``--result`` output file name, default ``<working dir>/results.json``

Tested on mac and win.