Rename “showdown” to “markdown”

If we want to change markdown implementations in the future, we’ll bump the major version of the package.

docs/.meteor/versions

@@ -28,6 +28,7 @@ json@1.0.0
 less@1.0.8
 livedata@1.0.9
 logging@1.0.3
+markdown@1.0.1
 meteor-platform@1.1.0
 meteor@1.1.0
 minifiers@1.1.0

packages/markdown/package.js

@@ -1,7 +1,5 @@
 // Source: https://github.com/coreyti/showdown
 
-// XXX rename to 'markdown' and credit showdown some other way?
-
 Package.describe({
   summary: "Markdown-to-HTML processor",
   version: "1.0.1"