Add link to Mercurial fork.
Add help documentation.
Ensure signs feature is present.
Highlighting customisation tweaks.

Highlight groups for signs:

- renamed highlight groups with `GitGutter` prefix, and following the
  naming style of the builtin groups `DiffAdd`, `DiffChange`, etc.

- using `highlight default link` to set default colours for signs.
  e.g. the plugin defines `GitGutterAddDefault` as green, and default
  (if `g:gitgutter_highlights`) links `GitGutterAdd` to it (which the
  user can customise).

Highlight groups for lines:

- added gitgutter-specific highlight groups for line highlighting
  (`GitGutterAddLine`, etc) so that this highlighting can be customised
  independent of `DiffAdd` et al (which these default link to).

- line highlighting toggling now only changes the linehl attribute.

  The `linehl` attribute is toggled in `s:update_line_highlights`,
  which separates concerns more clearly.  I'm thinking ahead to the
  possibility of the sign text being configurable, and thinking it will
  be DRYer to keep these things separate.  Maybe it's just aesthetic
  though.

Bonus: resolved "eugh" ;)
Define sign column highlights. Add colours and signs section.
Extract method for clarity.
Construct variable on one line for clarity.
Easier customisation.
Document special case of deleted first line(s).
Enable prev/next hunk commands to take a count.
Remove accidentally-committed maps.
Prevent infinite loop on Windows gVim.
Improve fix to Windows gVim infinite shell loop.
Document count support for jumping between hunks.
Document required support for signs in Vim.
Place a dummy sign to keep sign column.

The sign column automatically appears when there is at least one sign
and disappears when there are none.  There's a small gap between
when vim-gitgutter removes its signs and adds new ones; during that gap
the sign column can flicker.

By adding a dummy sign before removing the real signs, we force the sign
column to remain.
Handle non-English locales.
Make always-showing-the-sign-column opt-in.

This is consistent with Vim's default behaviour.
Update signs across all buffers on FocusGained.
Add debugging section to README.