Upgrades Guzzle Version
- Upgrade Guzzle Version from 6.2.0 to 6.2.3, addressing HTTP_PROXY security vulnerability, CVE-2016-5385 fixed in Guzzle 6.2.1
- Adds "DELETE" methods to the the __call method of HarvestService to support the new DELETE methods in Harvest.
- Updates documentation to better describe how to use Harvest POST requests, like postCandidate and postApplication.
- Updates documentation to describe how to use Harvest DELETE methods.