add error code for test timeout errors

[boneskull]

other:

• addresses some issues with API documentation of errors, featuring an example of how to properly document the Error object returned by the create* functions in lib/errors.js
• adds a missing test for the id prop of Runnable

[coveralls]

Coverage increased (+0.06%) to 94.116% when pulling 02400bc on boneskull/timeout-error into 025fc2e on master.