Add istanbul for code coverage.

`npm test --coverage` generates results, but basic `npm test` remains unsullied.

Already very impressive coverage! I just want help keeping it up to snuff when I write tests for the middleware-y stuff I'm cooking up.

```text
----------------------+-----------+-----------+-----------+-----------+
File                  |   % Stmts |% Branches |   % Funcs |   % Lines |
----------------------+-----------+-----------+-----------+-----------+
   combohandler/      |       100 |       100 |       100 |       100 |
      index.js        |       100 |       100 |       100 |       100 |
   combohandler/lib/  |      93.4 |     88.89 |     90.48 |     94.29 |
      combohandler.js |     96.43 |     92.59 |       100 |     97.59 |
      server.js       |     81.82 |     66.67 |     66.67 |     81.82 |
----------------------+-----------+-----------+-----------+-----------+
All files             |     93.46 |     88.89 |     90.48 |     94.34 |
----------------------+-----------+-----------+-----------+-----------+
```