Seperate unit and integration tests.

This makes it easier to re-use the integration tests in a seperate gem that
optimizes parts of liquid with a native implementation.

Rakefile

@@ -8,7 +8,7 @@ task :default => 'test'
 desc 'run test suite with default parser'
 Rake::TestTask.new(:base_test) do |t|
   t.libs << '.' << 'lib' << 'test'
-  t.test_files = FileList['test/liquid/**/*_test.rb']
+  t.test_files = FileList['test/{integration,unit}/**/*_test.rb']
   t.verbose = false
 end