rename to http filter and rework of configuration parameters

[jsvd]

This PR renames the rest filter created by logstash community members and also splits the request parameter into its separate components: body, url, headers, query parameters, etc.

It also changes the testing framework to use docker and pull version numbers from elastic/logstash.

TODO:

• [x] rename rest to http
• [x] break request into url, headers, parameters, method and body
• [x] remove sprintf options
• [x] handle sprintf vs non sprintf headers + parameters + body
• [x] documentation

closes #1, #2, #3

[guyboertje]

Tested gem in LS 6.4.1 with external APIs. GET and POST with json.

RLLYLGTM