Make a config file optional

With karma 0.9+ all parameter could be expressed with strings.
Now it is possible to put the hole config into the Gruntfile.

Only cave head is that the property `files` in a target has a special meaning. You have to wrap all into options object like this:
```js
karma: {
  options: { // this are parameters for all targets
    port: 9000,
    runnerPort: 9100,
    autoWatch: false,
    logLevel: 'error'
  },
  unit: {
    options: { // this are options for the target
      files: ['src/**/*js']
    }
  }
  e3e: {}  //etc.
}
```
grunt merge the options automatically.