Allow CVSS Score for severity if desired.

A setting inside models/db.py.sample allows users to use f_cvss_score as
the criteria for any severity-related function. To use change
settings.use_cvss to True.
more cvss preparedness
fixed exploited button in vulinfo_by_vulnid
Merge pull request #37 from grutz/tag_exploitable

fixed exploited button in vulinfo_by_vulnid
Merge pull request #38 from grutz/use_cvss

Support for f_cvss_score as primary severity