Add dictdiffer class to objects

As i had issue to add it as a submodule, i decide to copy the dictdiffer
code inside our object directory, i add in comment the source url and the
licence (MIT)
I made a small test implementing the __eq__ method for Reportiem
add __ne__ to vuln.py +pep8
improve RepportItem diff
rename vuln.py in reportitem.py
improve test coverage
correct constructor test ++ coverage
start diff of Host
rename class NessusHost as NessusReportHost

renamed:    libnessus/objects/host.py -> libnessus/objects/reporthost.py
implment diff for NessusReportHost object
finalise diff oh NessusReportHost object

TODO  more testing...
Update Readme for coverage badge :)