Multiple Admin improvements

Can now use markdown syntax in notes (bug 971584)
  Rename Note.html field to Note.note
  Backwards compatible with html markup
  Use AdminPagedownWidget for live previews in admin
Use AdminDateWidget for Release.release_date (bug 948726)
Add searching, sorting (bug 948723)
Merge pull request #17 from jgmize/markdown-notes-bug-971584

Multiple admin improvements