Child logger inherits 'pid' field.

[serhalp]

When a custom pid is configured on a logger, child loggers should inherit its value. Previously child loggers would get the default value from node's process.pid.

Fixes https://github.com/trentm/node-bunyan/issues/538.

cc @bobzoller

[serhalp]

Here's the requested change log line. I just copied the one for the hostname fix and substituted pid:

[pull #539, issue #538] Fix LOG.child(...) to not override the pid field of the parent. A use case is when one manually sets pid to something other than process.pid. (By github.com/serhalp.)

[serhalp]

bump @trentm