Simplify

Would you be interested in the following patch? Unfortunately original flask_injector is not enough for my applications so I had to make some changes and ended up changing quite a lot of code. I'm totally fine with keeping my changes in separate repository but I wouldn't want to fork without trying to have my changes merged first. :)

Here's what this patch does:

* simplifies the design, removes FlaskInjector and InjectorView
* sticks to "native" flask views (function views and Flask class-based views), classed-based views are passed the dependencies as constructor arguments
* allows injecting into ``before_request``, ``after_request``, ``request_tearing_down`` handlers and context processors
* adds some tests