Convert slashes in titles to dashes when migrating from WordPress

When a post has a title that contains a slash, such as 'This is my cool
blog post part 1/2', convert the slash to a dash so that the post
filename is created correctly.

Fixes issue #680

test/test_wordpressdotcom_migrator.rb

@@ -0,0 +1,9 @@
+require 'helper'
+require 'jekyll/migrators/wordpressdotcom'
+
+class TestWordpressDotComMigrator < Test::Unit::TestCase
+  should "clean slashes from slugs" do
+    test_title = "blogs part 1/2"
+    assert_equal("blogs-part-1-2", Jekyll::WordpressDotCom.sluggify(test_title))
+  end
+end