Added new optional argument to random for gifs on a subject

I've added an optional argument for getting random gifs (`Giphy.random`) so that random gifs can be found from a specific tag. I noticed it in Giphy's API but not in yours so I thought I'd try my hand and add it :smiley: 
Sounds good, I'll update the pull request!

> On 23 Jul 2014, at 21:45, Sebastian Sogamoso <notifications@github.com> wrote:
> 
> In lib/giphy/client.rb:
> 
> > @@ -34,8 +34,8 @@ def screensaver(tag)
> >        get('/screensaver', tag: tag)
> >      end
> >  
> > -    def random
> > -      get('/random')
> > +    def random(options={})
> I don't think we need to receive more than one argument here, optionally receiving a tag would be enough, making the interface simpler. What do you think?
> 
> —
> Reply to this email directly or view it on GitHub.