Adds an option to get "rspec like" output leaving only the failure messages

While the current output is awesome for continuous integration, I wanted to be able to focus only on the failure cases, mainly when doing TDD, because I have a suite with ~300 specs and when something failed I found myself having to scroll a lot to find the error. 

So, I bluntly stole from rspecs output, the result is something like this:
![demo](https://f.cloud.github.com/assets/692077/2181769/534bb4fc-9765-11e3-81ef-d8a6658b8a1f.jpg)

That way you can have two grunt tasks, one to get instant feedback from the testsuite and another one to see all messages.

Somethings that I think are worth noting:

**1)** I really don't know if the name `silenceSuccess` is the best for this, but I couldn't think of something better.
**2)** I think the code could be improved by extracting all 

````javascript
if (!options.silenceSuccess) {
  grunt.log.write();
}
````

into something like `output = Output.createFrom(options)` object that knows how to display the information to the user, that way all the conditionals can be removed (for example, doing `output.name(suite)`, `output.failures(suite)`) but I don't know if it's a good idea and I didn't wanted to change the code that much.
**3)** I changed the check mark to a dot because the terminal didn't play nice with it.
**4)** Lastly I wanted to test this feature but I got a bit lost in the project and couldn't find the specs. If tests are desirable and someone can point me in the right direction I'll be happy to add them.

Thanks!