gzip support .. please document it

[mafar]

Hi,
After going through solutions like ones suggested at #270

I found that frisby already supports gzip by adding {gzip: true} in get() or post() methods.
This is a very key feature and should be documented as other solutions are very ugly which involve unzipping in .after()

Working Sample:

//seems to work well in my tests
.get('http://SOMEURL', {gzip: true})
  .expectStatus(200)

Usage:

  frisby.create('GET JSON data from an endpoint')
  .get('http://SOMEURL', {gzip: true})
  .expectStatus(200)
  .expectHeader('content-encoding', 'gzip')
  .expectHeader('Content-Type', 'application/json')
  .expectJSON({ 'url': 'SomeValue' })
.toss();

[sasikanth]

Pull request for documentation added at vlucas/frisby-site#8