Add a gemspec and support loading ffi as a gem

Rakefile

@@ -1,6 +1,12 @@
+require 'rake/testtask'
+
 ROOT = File.dirname(__FILE__)
 
-task :test do
-  $:.unshift File.join(ROOT, "lib")
-  $:.unshift File.join(ROOT, "test")
+Rake::TestTask.new do |t|
+  t.verbose = true
+  t.test_files = FileList["test/**/t_*.rb"]
+end
+
+task :gem do
+  sh "gem build netlinkrb.gemspec"
 end