Introduce smarter sign management
README: update
Make sure b:sy_path is always set

References #136.
Introduce hunk text object
Doc: add remark about ]c and [c
Execute FocusGained action for all windows

References #113.
Add finer grained control over signs and highlights

I wanted to be able to customize signify in a way that wasn't possible
with the current set of options.  This commit adds 2 new options and 2
new highlight groups.

Specifically, we can now

1.  differentiate between changes, deletes AND signs that have both
    with the `signify_sign_changedelete` option.
2.  disable showing the number of deleted lines in the sign column with
    the `signify_sign_delete_use_count` option.
3.  highlight lines with changes and deletes specifically with the
    `SignifySignChangeDelete` highlight group.
4.  highlight the first line differently with the
    `SignifySignDeleteFirstLine` highlight group.

All of these default to existing settings/highlight groups, so
everything should continue working the way it does now.

--------------------------------------------------------------------------------

For those that are curious, I wanted to configure signify to show no
symbols in the sign column, and to just use the highlight groups to give
me all the information I needed.  I am able to achieve that now with the
following settings and the Solarized colorscheme:

let g:signify_sign_add = "\<Char-0xa0>\<Char-0xa0>"
let g:signify_sign_change = "\<Char-0xa0>\<Char-0xa0>"
let g:signify_sign_changedelete = "__"
let g:signify_sign_delete = "__"
let g:signify_sign_delete_first_line = "‾‾"
let g:signify_sign_delete_use_count = 0

hi SignifySignDelete term=bold,underline cterm=bold,underline ctermfg=1 ctermbg=12
hi SignifySignDeleteFirstLine ctermfg=1 ctermbg=12
hi SignifySignChangeDelete term=bold,underline cterm=bold,underline ctermfg=1 ctermbg=3

You can see it in action here:

https://twitter.com/_bentomas/status/486266117204090880
Introduce g:signify_sign_show_count

This option replaces g:signify_sign_delete_use_count and works for both,
delete and change/delete.

Referenes #125.
Doc: add g:signify_sign_show_count

References #125.
Make use of SignifySignDeleteFirstLine

Closes #137.
Use 'git -C'
Revert "Use 'git -C'"

This reverts commit 8fa7aea4b3b4c99891f2907828d4b99be6dee100.

Although -C was introduced in git 1.8.5, over a year ago, it still might be
too new for some of us.