Add MimeType to Note and allow Markdown content

* Introduce a "mimetype" field on the Note
* can be "application/x-tomboy-note" (for old-style xml notes) or
  "application/x-tomboy-markdown" for markdown based notes
* Bump note version to 0.4
* Do not close the passed in stream in XmlNoteWriter
Fixes for Issue #41

Always check the Server URL for ending forward slash