Added option to disable unicode in vimrc.

Unicode is enabled by default if the file encoding is UTF-8. Added
option to override this behavior by setting the following flag in the
user's vimrc:

g:notes_unicode_enabled = 0
highlight n/a todo items in the form: [n]
Option to highlight note names on word boundaries (pull request #67)
Try to resolve issue #62: Apostrophes are turned into curly quotes
Resolve pull request #56 (clarified intention of code)
Bug fix for issue #68 (silence E403)

See issue #68 on GitHub:
  https://github.com/xolox/vim-notes/issues/68
Change ASCII style bidirectional arrows to Unicode
Add logging to :SearchNotes in order to debug issue #53 on GitHub

See issue 53 on GitHub:
  https://github.com/xolox/vim-notes/issues/53
Don't sync title for shadow notes.

See https://github.com/xolox/vim-notes/issues/55
Merge pull request #69: Don't sync title for shadow notes

This also resolves issue #55.
First attempt* at support for notes in nested directories (issue #71)

* This might be a little rough around the edges, but it seems to work
  pretty well, so I currently don't see any reason not to make this the
  default. Some details I will have to think through a bit more, but
  they will no doubt become apparent once the feature is released.

  This change requires the latest version of vim-misc.
Merge pull request #49: Add option to disable Unicode
Add word boundary to Vim command highlighting (issue #59)
Add dash to 'iskeyword' so it can be used in tags (issue #61)
Highlight inline code fragments (issue #60)
Clarify intent of notes_title_sync option in readme (issue #23)
highlight paths containing BASH variables
Merge branch 'master' of https://github.com/xolox/vim-notes

Conflicts:
	syntax/notes.vim