client class should be replaceable

[nilsnolde]

The main advantage of this package IMO is the parsing of the response. We could re-use this in our valhalla QGIS plugin, but for that to work we need to be able to replace the client with a custom one (not using the requests package, but QgsNetworkAccessManager). That's a little bit of a bigger job of refactoring to make it possible:

• change the pattern to not have a base Router class but rather a client class which can get passed to the individual router classes
• the new (default) client class should be an abstract base class (similar to Router right now) which one can inherit from to build a new client class

should be doable in a backwards-compatible way.

[nilsnolde]

landed with #25