Added reactive versions of Activities and Fragments

Subclasses of the common `Activity` and `Fragment` classes, extended so you can easily get an `Observable<LifecycleEvent>`.

I wanted to get the ball rolling on this discussion (since it's the second part of #12, as I viewed it). I am fully aware this may be a controversial move. :P

I tried working up [a version](https://gist.github.com/dlew/33b650bd8ef3d360ff7d) using `ActivityLifecycleCallbacks`, but it felt too weird to have one solution for `Activity` and another for `Fragment`. Also, watching #84 has caused me to realize there are multiple places where being able to hook into the `Activity` or `Fragment` directly will save a lot of boilerplate.

I purposefully chose to not rename the subclasses. That way, when you extend, say, `Activity` the package resolution dialog will show this version as well.

One conspicuously missing extension is `ActionBarActivity`, since it depends on #83.