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
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)
Bug fix for issue #77: Error when deleting note when there are no buffers
Disable special key mappings inside {{{ snippets }}}

This is my first serious attempt to resolve issue #79 and issue #83.
I was originally worried that this might have serious impact on typing
speed because the <expr> mappings now have to search back through the
current buffer to determine their context.

In practice the slowness isn't that bad and it *is* really nice to type
regular quotes inside code blocks. No doubt this will become a performance
problem for someone editing 10.000 line notes, but we'll cross that bridge
when the get there ;-)
Trigger BufWritePre/BufWritePost autocmd events in xolox#notes#save()

It's hard to estimate the consequences of this change, but there's really
only one way to find out, hence I'm committing the change. The previous
behavior was a bug anyway, it's just that it could have been hiding a lot
of other bugs and incompatibilities until now... I'm keeping my fingers
crossed ;-)

See also issue #88 on GitHub:
  https://github.com/xolox/vim-notes/issues/88
Start using 'syntax sync fromstart' until we find a better way ...

See also issue #2 on GitHub:
  https://github.com/xolox/vim-notes/issues/2
Document usage of multiple platforms (issue #85)

See also issue #85 on GitHub:
  https://github.com/xolox/vim-session/issues/85