few facelift and botox injections for libnmap

v0.4.5, 06/04/2014 -- minor fixes and botox injections
                    - Added "incomplete" argument in NmapReport.parse()
                       in order to enable parsing of incomplete or interrupted
                       nmap scans. Could be useful to use with a background
                       scan to parse incomplete data blocks from callback
                       function (thanks @Sibwara for the idea).
                    - Fixed bug when NmapReport.summary is empty
                    - added NmapReport.endtimestr
                    - remplaced ElementTree by cElementTree (performance)