Add static method definitions to the SlackHTTP class

* Resolve strict warnings about calling non-static methods statically
* Convert a collection of curl_setopt to curl_setopt_array
* Convert a couple of booleans to lowercase to match the codebase
Update method signature of SlackServicePlugin::onHook

This solves an strict standards warning with the declaration of the
github_commits::onHook not matching the parent method.
Merge pull request #4 from there4/issue-2-strict-warnings

Fix issue #2 by resolving a couple of strict warnings