Coverage tests

Some initial coverage tests.

As you note, Promise$all and Promise$race are the main uncovered functions.  With this patch, running 

     $ npm run-script cover

will run the full aplus test suite, plus a very few handwritten tests for the ES6 portion of the spec, yielding the following coverage stats

File                        |   % Stmts |% Branches |   % Funcs |   % Lines |
----------------------------|----------|-----------|-----------|-----------|
   native-promise-only\     |       100 |       100 |       100 |       100 |
      test_adapter.js       |       100 |       100 |       100 |       100 |
   native-promise-only\lib\ |     80.43 |     73.49 |     70.45 |      80.9 |
      npo.src.js            |     80.43 |     73.49 |     70.45 |      80.9 |
All files                   |     82.27 |     74.12 |        74 |     82.74 |