Fix bug in Safari on teach.html #253

[6stringbeliever]

This fixes issue #253. Safari was confused by using relative positioning with the center class and absolute positioning with the center-btn class. We can fix by removing the center-btn class altogether and fixing the spacing in the teach-top and teach-bottom classes. Changed white-space to normal on the buttons because otherwise the text won't wrap and the button will spill out of the box at the skinniest column size.

Tested in IE11 and latest Chrome, Safari, and Firefox on Mac.

[ondrae]

Much better, thanks @6stringbeliever