Fingerprint object in knowledge base

[andresriancho]

Add an object that inherits from info() and represents a system fingerprint. We should have different classes of fingerprint objects, one for OS, other for HTTP daemon, other for programming language, other for programming framework, etc. These should be stored in the KB by the infrastructure plugins that do this job. In the future this could be used by the core to cross this info with an XML file that says "PHP version X.Y.Z has vulnerability CVE-12345".