Fix #472 Use native http/https.Agent

The 'keep-alive-agent' module is not functional on Node v0.11.6 and
above due to http agent class refactoring.  While this breakage is
arguably a bug in Node, there's also no need for a keep alive agent in
Node v0.11 and above, since http keepalive is supported natively now.

Sniff for the Node version, and choose the appropriate agent and configs
accordingly.
test: update client malformed test for node v0.11

The 'spaces in path' error is now caught early in the client,
since it's never valid HTTP.  Update the test to handle this.
version bump
Fix connecting socket detection
Don't override response code in res.send(code, errorObj)
If you invoke res.send() with an error that has a statusCode attribute,
that will be used, otherwise a default of 500 will be used
(unless you're using res.send(4xx, new Error('blah)))."

http://mcavage.me/node-restify/#Error-handling
Merge pull request #494 from rysenko/master

Fix connecting socket detection
Merge pull request #495 from 13W/customCode

Don't override response code in res.send(code, errorObj)
Merge pull request #492 from 13W/master

"false" disables retry
Add test cases to static plugin to demonstrate failure with certain regular expressions, see project issue #496
Merge pull request #497 from iwarren/master

Add test cases for serving static files
[minor] lint/style/changelog update
#496 - rollback to 2.6.0-esque static plugin
ensure have a reasonable client_req serializer (see similar #358)

Without this, a logger at trace level to the restify client will crash
(on eventual memory exhaustion) attempting to log the raw `client_req`.
style
Merge pull request #498 from trentm/client_req_log_boom

 ensure have a reasonable client_req serializer (see similar #358)
Merge pull request #475 from isaacs/gh-472

Fix #472 Use native http/https.Agent