Merge pull request #273 from geisesteert/patch-2

Add nmap package

packages/nmap.rb

+require 'package'
+
+class Nmap < Package
+  version '7.31'
+  source_url 'https://nmap.org/dist/nmap-7.31.tgz' # Software source tarball url  
+  source_sha1 'ccf1bb34463f39a645d9a924ce9e3c9e15eefbbf'
+
+  depends_on 'buildessential'
+  
+  def self.build                                                  # self.build contains commands needed to build the software from source
+    system "./configure" 
+    system "make"                                                 # ordered chronologically
+  end
+  
+  def self.install                                                # self.install contains commands needed to install the software on the target system
+    system "make", "DESTDIR=#{CREW_DEST_DIR}", "install"          # remember to include DESTDIR set to CREW_DEST_DIR - needed to keep track of changes made to system
+  end         
+end