use karma's error formatter

The error stack trace will be filtered, so that instead of timestamped URLs it'll contain resolved source paths. Also indentation of the stacktrace is corrected.

with karma formatter:

```

FAILED TESTS:
  high level API
    .split()
        ✗ generates highcharts options with group counts
          PhantomJS 1.9.7 (Linux)
        AssertionError: expected { Object (xAxis) } to deeply equal { Object (xAxis, series) }
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/karma-sinon-chai/node_modules/chai/chai.js:918
            at assertEql (/home/artm/src/dpi/dashboards/dash.js/node_modules/karma-sinon-chai/node_modules/chai/chai.js:1354)
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/karma-sinon-chai/node_modules/chai/chai.js:3522
            at /home/artm/src/dpi/dashboards/dash.js/src/js/spec/seq_spec.js:160
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4336
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4724
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4815
            at next (/home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4649)
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4659
            at next (/home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4597)
            at /home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:4626
            at timeslice (/home/artm/src/dpi/dashboards/dash.js/node_modules/mocha/mocha.js:5733)
```

Same error without:

```
FAILED TESTS:
  high level API
    .split()
        ✗ generates highcharts options with group counts
          PhantomJS 1.9.7 (Linux)
        AssertionError: expected { Object (xAxis) } to deeply equal { Object (xAxis, series) }
    at http://localhost:9876/base/node_modules/karma-sinon-chai/node_modules/chai/chai.js?1381402002000:918
    at assertEql (http://localhost:9876/base/node_modules/karma-sinon-chai/node_modules/chai/chai.js?1381402002000:1354)
    at http://localhost:9876/base/node_modules/karma-sinon-chai/node_modules/chai/chai.js?1381402002000:3522
    at http://localhost:9876/base/src/js/spec/seq_spec.js?1398668518000:160
    at http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4336
    at http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4724
    at http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4815
    at next (http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4649)
    at http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4659
    at next (http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4597)
    at http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:4626
    at timeslice (http://localhost:9876/base/node_modules/mocha/mocha.js?1390426114000:5733)
```