always fail on bad tokens

bug discovered in parser via fuzzing

it's the last one I could find with very-non-sophisticated fuzzing, so I guess this closes #28
Merge branch 'master' of github.com:uniphil/expression-compiler

Conflicts:
	parse.js
	test.js
add coveralls coverage test with istanbul
Merge pull request #32 from uniphil/coverage

Add code coverage report

woo hoo coveralls seems to work!