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.
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
CORS typos
ssl connection freezes
fix empty serializers
Merge pull request #500 from 13W/ssl-timeout

ssl connection freezes
Merge pull request #501 from 13W/serializers

fix empty serializers
Update server.js
Update CHANGES.md

Add ciphers in https server options
Update package.json
Merge pull request #508 from BrianPin/master

Update server.js
requestTimeout support
minor client.test.js cleanup
#502: server not emitting "request"
Starting requestTimeout after socket connects
Merge pull request #499 from gergelyke/master

CORS typo fixes