vfeed lookup added in cves library

Lookup can be enabled using vfeedlookup=True when instantiating
the method. The returned value will contain if the vfeed attribute
if this exists.
vfeed dict is not merged when it's a string
Return None if CVE requested doesn't exist.
Add cpe name|ranking|vfeed lookup in the web interface.

Now the default template is displaying all the know records
for a CVE including CPE, ranking and vfeed references.