add border and margins to the even-number exercises to match the odd ones.
change button appearances to better match functionality
adds some styles to end-of-chapter exercises (issue 193)
more styling of end of chapter exercises
Merge pull request #202 from isaacdontjelindell/issue-193

Give the questions at the end of the chapter a background color
More skulpt upgrades

minor fixes from big merge with codeskulptor, and all unimplemented builtins have skeletons that throw an unimplemented exception
reformat line numbers as part of errors.   Add information about the unimplemented exception
Better formatting of error messages inside of activecode
Encapsulate solutions that are inside reveals in a nicer formatting, including a border.