Added core-pages to change when exam-card fab is tapped

The core-pages element was implemented to handle the
changing of the page content when an exam-card's fab is
tapped. A binded variable 'selectedPage' is passed to all
exam-cards that is set to 1 when the on-tap event is
fired.
Changed core-pages to core-animated-pages

To have cool looking transitions when changing pages.
Added fab as a back button

A fab was added to act as a back button to return to
the selection of topics when answering exam questions.
Wrapped header in a responsive div

So that the header won't stick all the way to the left
on very large screen sizes.
Changed slide transitions to cross-fade

Slide transitions are broken, they are causing the fab to
temporarily move by several pixels to the left and right
during transitions.