New unittest library and driver script for NSE testing

Please read the documentation. This is a way to add unit testing to NSE
libraries (not scripts, yet). Please add tests to your libraries!
Examples to come in further commits.

scripts/unittest.nse

@@ -0,0 +1,42 @@
+local stdnse = require "stdnse"
+local unittest = require "unittest"
+
+description = [[
+Runs unit tests on all NSE libraries.
+]]
+
+---
+-- @args unittest.run Run tests
+-- @args unittest.tests Run tests from only these libraries (defaults to all)
+--
+-- @usage
+-- nmap --script unittest --script-args unittest.run
+--
+-- @output
+-- Pre-scan script results:
+-- | unittest:
+-- |_ All tests passed
+
+author = "Daniel Miller"
+
+license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
+
+categories = {}
+
+
+prerule = function() return stdnse.get_script_args("unittest.run") end
+
+action = function()
+  local libs = stdnse.get_script_args("unittest.tests")
+  local result
+  if libs then
+    result = unittest.run_tests(libs)
+  else
+    result = unittest.run_tests()
+  end
+  if #result == 0 then
+    return "All tests passed"
+  else
+    return result
+  end
+end