Python3 compatibility with iterators.
[doc] Fix typos in docstrings.
[wapiti] Fix bug where a ranking could be None.

Specified a default ranking value to UNKNOWN when trying to retrieve the one
from the signatures database in case the matching fails.
[metasploit] Support to Metasploit.
[report] Fix ValueError in get_highest_ranking

Fix ValueError: max() arg is an empty sequence in get_highest_ranking that
could be triggered when self.vulns is empty.
[tools] Fix IndexError in parse().

When calling _recursive_find function, first test if a result has been found
and only then keep the first one.
[wapiti] Fix typo in docstring.
[ptp] Modular calls to parse and is_mine functions.
[ptp] Add support to Metasploit.
[tests] Add unit tests for Metasploit ftp anonymous plugin.
[metasploit] Enhance support of scanner plugins.
[ptp] Fix undefined behavior in get_highest_ranking

Force ptp to return `UNKNOWN` ranking values if no report has been found when
calling `get_highest_ranking` function.
[metasploit] Enhance signatures for scanner modules.
[metasploit] Add unit tests for scanner plugins.
[metasploit] Risk re-evaluation for some plugins.

Thanks Tedi Heriyanto for the help ;)
[tests] Update unit tests for metasploit plugins.
[metasploit] Add signatures for DoS and Exploits modules.
[ptp] Fix typo in docstring.
[doc] Update metasploit section.
Update readme.