Tweaks to main view

Adjusted the main view.  Answer buttons are now aligned center-bottom,
and question text has a max size of 8 lines (may be adjusted later) and
set scrollable in case the text is too big.  Also, created a
"blackMedium" text style in res/values/styles.xml.  We should use this
for all "medium" size text values to keep a consistent style (like CSS)