Improve scanning performance and reduce false negatives

scan all the machines from top-1m.csv file, wait for completion
of all jobs

i=1 is an off-by-one-error

support top-1m.csv files with arbitrary number of sites

run scans for many hosts at a time, but don't run more than
specified amount

in case where default domain name doesn't resolve or doesn't have
port 443 open, retry with www. prefix
Merge pull request #14 from tomato42/scan-improvements-02

Improve scanning performance and reduce false negatives