#83716236: Match blocks proportional scaling of images

Solves [original request](https://www.pivotaltracker.com/story/show/83716236) to scale images in match blocks without changing aspect ratio.  Also vertically centers images, and sets them on a white background with uniform colored borders for a cleaner look.  Colors updated a bit to match [this color guide](https://s3.amazonaws.com/uploads.hipchat.com/65395/780341/kCxI5OwDMpdpdAz/Screen%20Shot%202014-10-14%20at%205.38.53%20PM.png).  Tried to improve usability by adding a border color change and cursor change on rollover of an interactive tile, and adding a darker dotted outline to valid drop zones while dragging a tile.
Nope, pretty sure we don't support IE8 in Code Studio (and show a big warning message for IE8 visitors), I believe just on code.org/* we do