Use httpie for tutorials

Closes #2068

I've worked on this today for the [24pullrequests](http://24pullrequests.com/dashboard), in case you want to merge it later on after a decision is made.

Httpie has replaced all the examples in:

* Quickstart
* Tutorial

The Quickstart and tutorial sections also help users install it, and give reasons why they should use it.

I decided not to replace the ```curl``` examples on the authentication pages. This is because httpie only supports basic and digest authentication unless you install extra things, which would just take too much time for the user and detract from the purpose of the documentation.

I've also changed the example output to match the output of httpie, which uses a lot of new lines and is quite verbose. I'm not sure if we want to keep it on a single line to save space or try and stick to what the user should expect to see. I vote for the later in this case.
"Or, using the [httpie][httpie] command line tool..."