Implement EngineActivityRequest Call

[mdaines-r7]

Implement the EngineActivityRequest call to the Nexpose API, which provides a list of current scan activities for a specific Scan Engine.

[mdaines-r7]

With a parser written for ScanSummary DTDs, it will apply to all of the following calls: ScanActivityRequest, ScanStatisticsRequest, SiteScanHistoryRequest, and EngineActivityRequest. These should all be refactored to use the same response object.

[mdaines-r7]

Pushed and updated gem version to 0.1.3. Will push to rubygems after soliciting comments.

Workflow should now be like this:

scans = nsc.engine_activity(20)
scans.each do |scan|
puts scan.scan_id
puts scan.status
puts scan.nodes.live
puts scan.vulnerabilities.vuln_exploit.count
end