Server/CLI: Optimize console output

Only log screenshot URLs for failed tests, still always create them,
can always be accessed on browserstack.com/automate.

Degrade the "Exiting" logs to debug, not needed in normal operations.

Update logs for finished workers to use less color, slightly more compact format.

Fix bug in formatTraceback, where a falsy actual or expected value would cause
nothing to be printed.

Updates tests/test.rby to match the new finished console output.
Server: Remove unnecessary worker check
Server: Remove unnecessary worker fallback

The fallback will never apply, since the previous if already establishes that
workers[query._worker_key] is defined.
Build: Add npm-test script, run mocha and jshint (new), fix lint issues

Adds .editorconfig and .jshintrc for better code consistency.

Run jshint along with moch tests using `npm test`. Fixes tons of lint
issues, like double quotes instead of single quotes (there were more
single then double quotes, so I went with single quotes).
README: Overhaul everything, making usage more clear, add contributing

Adds CONTRIBUTING.md specifically for contributors, not users. Makes
installing from npm the primary usage example. Compress the default
configuration example. Removes a bunch of newlines to make editing
easier. Uses better formatting for parameter lists.
Merge pull request #97 from jzaefferer/optimize-console-output

Optimize console output