implement Phrase::Api::Client#fetch_translations

Hey Guys.

I was using an instance of Client in a script and realized that the ```GET /translations``` endpoint wasn't available.
I wound't respect myself if I called ```JSON.parse(my_client.send(:perform_api_request, "/translations", :get))```
So I created a ```fetch_translations()``` method that accepts a hash of parameters to be passed on.

##### FInally

* You'll see my text editor also stripped out whitespace.

* I added an unimplemented rspec test for it because it looks like your VCR is set to record tapes on localhost.
Let me know if there's a better way.