Moved valid_post_data to post#valid?

used this method to only write valid posts. Hoping to put this in
PostCollection soon.
Changed PostCollection to act as an Enumerable

PostCollection held logic to create Post obects from the wp_export file.
However, in Migrator all it did was pull the array of posts.  This array
later was filtered to non-attachment and valid posts.  I moved the logic
of filtering to the PostCollection and implemented it as a immutable
Enumerable.  Now PostCollection behaves like a primitive array.
Moved the content methods in migrator to Post
Moved the options for markdown and include fields

The options of markdown and include fields will be passed to
Post#initialize and be used inside Post.  This helps to not having to
pass them around so much.
Moved the migration logic out of Post and into MiddlemanPost
Merge pull request #24 from natedavisolds/master

Where do responsibilities belong