adding publisher datetime and updating nvd feed download

[ErikThoreson]

nvd.go and xml.go: added publisher datetime from xml feed and updated the XML feed download to complete before moving onto downloading the next feed

Reason:
The NVD site frequently resets connections when downloading the feeds if they are not downloaded in a timely manner. When all the feeds are requested at once but then not downloaded immediately the NVD site will reset the connection. It is more reliable to download each after making the request instead of opening all the connections then downloading them one at a time.
Also added is the Publication Datetime from the XML. This field is useful for compliance as many vulnerability programs for PCI base their remediation priority off this date field.

Example feed error:
{"Event":"could not decode NVD data feed","Level":"error","Location":"nvd.go:93","Time":"2017-08-25 18:48:15.515078","data feed name":"2014","error":"read tcp 192.168.10.9:54476-\u003e129.6.13.177:443: read: connection reset by peer"}

NOTE: This is a ref to this PR. Just a bit more cleaned up. #496

[reasonerjt]

@jzelinskie @ErikThoreson
Will this be cherry picked to release-2.0 branch?
I'm seeing the connection reset issue when using Clair 2.0.4