Support maxSockets

[thinkingserious]

Issue Summary

See sendgrid/sendgrid-nodejs#270

Technical details:

• node-http-client Version: 2.X
• Node.js Version: 4.1.2

[yanosh-igor]

It will be very useful feature. Waiting for it..

[jaydp17]

@thinkingserious looks like you are no longer using https://github.com/sendgrid/nodejs-http-client in https://github.com/sendgrid/sendgrid-nodejs.

[thinkingserious]

That's correct @jaydp17.

[valtlfelipe]

I believe this has already been implemented in #15.

Or is this issue related to implement it on https://github.com/sendgrid/sendgrid-nodejs/blob/master/packages/client ? If yes, I can take it.