Modal for screenshot; adds map tiles & key

Builds on #86 and replaces #85.

This modifies the screenshot link to match the requested design (placing it in the lower right). When a screenshot is taken, it is displayed in a modal (solving certain IE issues).

If not in IE (where security issues prevent this), the code steps through each image of the background tiles, drawing them to the canvas (some deferred action is required due to cross-origin issues). Then, it draws the svg as before. Then, draw the key and add some strings to indicate the value of the selectors. Finally, open the drawn image in a modal for saving.

We'll eventually want the proper font size, etc. but this is a working prototype. We may also be able to load the tile images via ajax to avoid IE's concerns.