Add asset_scan_history method

[gschneider-r7]

This is convenient for a small number of assets, but performance does not scale well for many assets. Something like ~25 assets took 30+ seconds to return using the below example. I was inspired by this community discussion: https://community.rapid7.com/thread/7780

Example usage, retrieving the last scan for assets with 'centos' in their hostname:

assets = nsc.filter(Nexpose::Search::Field::ASSET, Nexpose::Search::Operator::CONTAINS, 'centos')

last_scans = []

assets.each { |asset| last_scans << nsc.asset_scan_history(asset.id).first }

last_scans.each { |scan| puts "Asset: #{scan.asset_id}, IP: #{scan.ip}, Name: #{scan.host_name}, Scan: #{scan.scan_id}, Site: #{scan.site_id}, Engine: #{scan.engine_name}" }

Example AssetScan object:

#<AssetScan:0x007f80c338a8b0
 @asset_id=1777,
 @end_time=2015-11-13 15:34:50 -0800,
 @engine_name="Local scan engine",
 @host_name="centos-linux",
 @ip="192.168.1.2",
 @os="Linux 2.6.32-71.el6.x86_64",
 @scan_id=238,
 @site_id=270,
 @site_name="My Linux Site",
 @vulns=12>

[houndci-bot]

Do not use Time.at without zone. Use one of Time.zone.at, Time.at.current, Time.at.in_time_zone, Time.at.utc, Time.at.getlocal, Time.at.iso8601, Time.at.jisx0301, Time.at.rfc3339, Time.at.to_i, Time.at.to_f instead.