Fix connection pooling behavior by reading responses from endpoints sooner.

The faster we read response from endpoint and close the body, the sooner it will be released back to the pool and reused.
In addition to that we improve copying by implementing optimized WriteTo method
Updated code review comments
Merge pull request #118 from mailgun/prof

Fix connection pooling behavior by reading responses from endpoints sooner
Build docker image using official golang as base
pin golang base image to 1.3.3