Documentation for Getting started with the Raiden API

[czepluch]

This fixes some small errors in Overview-And-Guide.rst and Rest-Api.rst

It's main focus is to write a user guide for the API endpoints in accordance to #668

[LefterisJP]

Made some fixes in the documentation and looks good to me.
We still need to adjust the Makefile and make sure that the documentation builds better. The documentation builds but there are errors and also files outisde the _build directory. Will try to take care of that myself.

@czepluch two notes about documentation in restructured text:

1. Your in document links with :ref: were fine but you need to have a blank line between the anchor and the title. e.g.:

.. _joining-existing-token-network:

Joining an already existing token network
=========================================

2. The external document links should not have the .rst extension or have the directory specified. It can be found by the sphinx document builder and it needs to not have an extensions since docs also get compiled to html and pdf. e.g.:

please see :doc:`Overview and Guide <Overview-And-Guide>

[LefterisJP]

Will merge this and further work can happen in other PRs.