activity: asks both webviews to stop loading when getting a new URL

First, this seems to help performance a lot, second we don't want the
event `onPageFinished` to be fired if the page took too long to display.
manifest: enfore the use of API 19 to get appropriate webview

While we could compile with just API 16, we would get an inappropriate
webview. Just ensure we use the most recent version. Hence, we are only
compatible with Android 4.4.
README: also says how to display logs from AndroidRuntime

They contain backtraces and other interesting debug stuff.
layout: use a view pager to transition between the two web views

The main advantage is that the view does not have a null size while
rendering. This should lead to better transitions.
build: ship android-support-v4.jar

The way to just use the one from the SDK is quite undocumented. Most
projects seem just to drop the JAR in the `libs/` directory...
activity: reorder import alphabetically
webview: fix scrolling

Due to previous change, we cannot rely on visibility for scrolling since
both webviews stay "visible". We use focus instead.
layout: replace `fill_parent` (deprecated) by `match_parent`
activity: make a "flip" transition between the two webviews