Change errant tabs to spaces
Auto set buffer filetype to markdown if checked as md in Simplenote

Read "systemtags" property in note to check if indicated as markdown in
Simplenote. If it is then set buffer filetype to markdown

Fixes #49
Toggle markdown flag in Simplenote to match current buffer filetype

Changes `update_note_from_current_buffer()` so that it sets the markdown
flag in Simplenote to match the filetype of the buffer. This means by
setting the filetype locally you can toggle the markdown status of a
note within Simplenote.

Also changes `create_new_note_from_buffer()` to set the markdown flag if
the buffers filetype has been set as markdown.

Tries to play safe with this approach and if for whatever reason the first
`note, status = self.simplenote.get_note(note_id)` fails then doesn't
try to set the system tags via:

`note, status = self.simplenote.update_note({"content": content,
				    "key": note_id,
				    "systemtags": note["systemtags"]})`

as note["systemtags"] would be undefined

Fixes #49