Add workspace (tex-pad?) to all exercises

tldr: A instantly parsing textarea. Go to this [sandcastle](http://sandcastle.kasandbox.org/media/castles/Merbs:texpad/exercises/absolute_value_equations.html) and click on "Show workspace" at the bottom of the page.

This is just a proof of concept as an alternative to the scratchpad. It adds the link "Show workspace" at the bottom of the page alongside "Show scratchpad". When clicked, it produces textarea that takes up half the width of the screen, which upon input, parses that input and displays the parsed tex on the other half of the screen. Atm, it defaults to the pseudo-tex that the `expression` answer-type reads in and feeds to KAS. It also provides a "Switch to tex" option, which is far less limited, but you need to know tex, and end each line with `\\`. The provided version has multiline support in the pseudo-tex format (adds the `\\` for you). There are definitely issues (for example, KAS can parse `1=1` but can't parse `1=1=1` or `3|x|`), but they can be fixed if you (collective) think this is worth pursuing.

As a rationale, 
- this is how I do my maths
- an algorithm can possibly read in their input, watch as they reduce line by line, and unobtrusively alert them if they made an error (forgot to distribute a -1, added incorrectly, etc.), with some time delay (or after they submit an answer).
- data aggregation (per problem) could look for common mistakes, which couldn't be derived from the user's guess alone.