Quadriga

* Added `quadriga.RequestError.message` attribute.
* Added `quadriga.RequestError.response` attribute which allows users to introspect the HTTP request response details.
* Travis CI builds will now halt on documentation build warnings and failures.

* Overhauled the API to reduce room for user error. Unfortunately, this comes with non-backward compatible changes (hopefully for the best reasons).
* Instead of declaring the default book while initializing `QuadrigaClient`, or passing in the order book name as an argument to methods, a new class [quadriga.client.OrderBook](https://quadriga.readthedocs.io/en/latest/api.htmlorderbook) was introduced to separate order book specific API, and enforce the user to explicitly declare which book to use. See the [documentation](https://quadriga.readthedocs.io/en/latest/overview.html) for more details.
* Added support for new major currencies (e.g. BTG, BCH).
* Added small improvements to the method signatures, for full list of changes please refer to the [API specification](https://quadriga.readthedocs.io/en/latest/api.html).
* Fixed a nonce bug.

* Added [contributing page](http://quadriga.readthedocs.io/en/master/contributing.html) in the documentation.

* Added `license` information in setup.py.

* Added support for Litecoin (order book `ltc_cad`).
* Added new method `set_default_book` to `quadriga.QuadrigaClient`.

* Corrected wrong order book name `eth_usd` to `eth_btc`.