Simplify Hello World apps and update docs (#28)

* Simplify Hello World apps
* Updated docs

docs/app-lifecycle.md

@@ -0,0 +1,12 @@
+# Application Lifecycle
+
+The app goes through these states:
+
+![app lifecycle state diagram](pics/app-lifecycle.png)
+
+Let's look at a scenario where an app launches another app:
+
+1. `first` app starts: `first.on_create()` -> `first.on_show()`
+2. `second` app starts: `first.on_hide()` -> `second.on_create()` -> `second.on_show()`
+3. `second` app stops: `second.on_hide()` -> `second.on_destroy()` -> `first.on_show()`
+4. `first` app stops: `first.on_hide()` -> `first.on_destroy()`