Reformat GitGutterGetHunkSummary's array

Update the array's format returned by GitGutterGetHunkSummary. 
From old format [0, 0, 0] into new format [+0, ~0, -0] to make it easier to understand when people integrate vim-gitgutter into vim-airline's status bar.

Before
![screen shot 2014-10-16 at 3 50 53 pm](https://cloud.githubusercontent.com/assets/3027146/4659831/9f351150-5511-11e4-97ad-2a52b9da2c8a.png)

After
![screen shot 2014-10-16 at 3 48 47 pm](https://cloud.githubusercontent.com/assets/3027146/4659838/ad9762fc-5511-11e4-932e-52276d0a5152.png)