add support for before/after hooks to support tracing outbound requests

[joshwilsdon]

This change adds new options to the create*Client functions which allow before
and after hooks to be called for every request made by the client. This allows
for implementation of a tracing system around requests. It also allows a before
hook to add additional headers so that remote servers are provided tracing
information.

It replaces an earlier PR #77 which attempted to do the same
thing in a slightly different way.

[coveralls]

Coverage decreased (-0.2%) to 84.453% when pulling e3903a8 on joshwilsdon:rfd-35-cls into c72cc34 on restify:master.

[coveralls]

Coverage decreased (-0.2%) to 84.595% when pulling ad08dba on joshwilsdon:rfd-35-cls into bbdeb79 on restify:master.

[coveralls]

Coverage increased (+0.4%) to 85.19% when pulling 7a44312 on joshwilsdon:rfd-35-cls into bbdeb79 on restify:master.

[DonutEspresso]

@joshwilsdon Sorry for the radio silence on this. This feature is something we'd use and I'd like to see this merged before the 2.x release.

A lot of movement has been occurring in this repo since you first opened this PR though, so it looks like it's a bit out of date. I'll take a shot at reconciling this against master to get it over the finish line. Appreciate the PR.

[joshwilsdon]

I don't know how I missed the update from @DonutEspresso but I was finally coming back to look at this and see that their PR is in better shape than mine at this point so closing this out in favor of #156.