Adds istanbul code coverage w/o external template

I'm not sure if this is desirable but I wanted to add coverage reports directly to `grunt-contrib-jasmine` w/o needing to configure another library in my Gruntfile.js.  This does that with little effort for config and little overall changes to the code.  

If a user just adds `istanbul:true` to their config like:

```js
jasmine: {
  mytask: {
    src: [...],
    options: {
      istanbul: true
    }
  }
}
```

The resulting output looks like:

```shell
Running "jasmine:istanbulTest" (jasmine) task
Testing jasmine specs via PhantomJS

 Player
   ✓ should be able to play a Song
   when song has been paused
     ✓ should indicate that the song is currently paused
     - should be possible to resume...
log: Console logging test
     ✓ should be possible to resume
   ✓ tells the current song if the user has made it a favorite
   #resume
     - should throw an exception if song is already playing...
log: Console logging test
     ✓ should throw an exception if song is already playing

5 specs in 0.112s.
>> 0 failures

=============================== Coverage summary ===============================
Statements   : 93.75% ( 15/16 )
Branches     : 100% ( 2/2 )
Functions    : 85.71% ( 6/7 )
Lines        : 93.75% ( 15/16 )
================================================================================
```