Add a doc page for application composition

As noted, in #73, we can already structure klein apps into parts that can be composed together.

Let's write add an example to the docs, since it's not super obvious how to do that.

I tweaked @glyph's example code into something I think may work:

from klein import route, run, Klein

class HelloApplication(object):
    app = Klein()

    @app.route("/")
    def hello(self, request):
        return "Hello!"


class MathApplication(object):
    app = Klein()

    @staticmethod
    def numberify(string):
        if "." in string:
            return float(string)
        else:
            return int(string)

    @app.route("/")
    def root(self, request):
        return "Math happens here."

    @app.route("/add/<a>/<b>")
    def add(self, request, a, b):
        return "{}".format(self.numberify(a) + self.numberify(b))

    @app.route("/subtract/<a>/<b>")
    def subtract(self, request, a, b):
        return "{}".format(self.numberify(a) - self.numberify(b))

    @app.route("/multiply/<a>/<b>")
    def subtract(self, request, a, b):
        return "{}".format(self.numberify(a) * self.numberify(b))

    @app.route("/divide/<a>/<b>")
    def subtract(self, request, a, b):
        return "{}".format(self.numberify(a) / self.numberify(b))

    @app.handle_errors(ValueError)
    def valueError(self, request, failure):
        return "Invalid inputs provided."


class Application(object):
    app = Klein()

    @app.route("/")
    def root(self, request):
        return "This is a web application composed from multiple applications."

    @app.route("/hello/", branch=True)
    def hello(self, request):
        return HelloApplication().app.resource()

    @app.route("/math/", branch=True)
    def math(self, request):
        return MathApplication().app.resource()


if __name__ == '__main__':
    application = Application()
    application.app.run("localhost", 8080)

Start by reviewing the existing documentation structure and the application-composition example provided in this issue. Add a documentation page explaining how Klein applications can be composed, using a corrected, runnable version of the example. Done means a newcomer can find and follow the example to understand composition.