Added buttons Call and Mail to People list.

The buttons work on Android so far.
Additional fields for Person view

Added url, building and headquarter fields to the People model.
In the Person view the url is opened by the button. The button
is shown only when the url is not null.
Isolated platform dependant code

Isolated the platform dependant code. All the platform dependant
javascript code has been moved to the js/[platform] and
plugins/[platform] directories. The functions used in the app
are added during runtime and allow a separation between the
architecture dependant code and the common code.

Removed Childbrowser plugin because it is no necessary anymore.