Fix min/max port option bug

The entire options object is being passed into portScanner.getPortRange
which fails all the checks and always returns 3000/4000. This
effectively ignores any custom port range options being sent in.

This fix simply extracts min and max port values from the options object
(if provided) and passes them in correctly as two separate arguments to
portScanner.getPortRange.
Added unit test for issue #48

Added a test to ensure that the internal getPortRange function call
correctly makes use of the provided ports in an options object. This
test breaks without the fix for min/max port options bug (issue #48).
Merge pull request #48 from brutaldev/master

Fix min/max port option bug