[META 382] Zero-configuration support

[AlexanderWert]

Implement zero-configuration support for the agent as described in: elastic/apm#382

SPEC issue: elastic/apm#401

[trentm]

• handle the rare case for "service.name" discovery to not error out if the $CWD of the process is not inside the directory tree holding the package.json file. Instead we'd change it to fallback to some default "unknown nodejs service" string.