chore: update jshint and move it to an npm script

[nfischer]

I think this is worth it because:

1. jshint is udpated, so we'll get whatever bug fixes there may have been as well as performance improvements
2. This is now an npm package script instead of part of run-tests.js. Although cutting down on code is nice, this really shines in that it exposes linting to the commandline (so you can lint without re-running tests if you choose), and it lets jshint figure out which files to lint, instead of using common.expand() to expand the * character for Windows
3. I removed most re-declarations of variables just because I know that's an issue for eslint, and I know we may migrate to use that in the future. This wasn't required by jshint, however
4. I made linting a bit stricter, and actually caught some confusing parts of the code
5. Linting now runs after tests. I think this is valuable if you want to first get your code correct, and then afterward fix it for stylistic consistency.

[nfischer]

@ariporad rebased this off master

[ariporad]

LGTM!