Use bg values from SignColumn for GitGutter{Add,Change,Delete}Default

Previously you used `NONE` as `guibg` and `ctermbg` values for the `GitGutter{Add,Change,Delete}Default` highlight groups. Combined with color schemes that use a non-`NONE` background color for `SignColumn` they don't match:

![before](https://cloud.githubusercontent.com/assets/474504/2623890/093af524-bd0f-11e3-97e9-0f43919df5b7.png)

This pull requests reads the current background values from `SignColumn` and uses them instead:

![after](https://cloud.githubusercontent.com/assets/474504/2623893/2a3a526a-bd0f-11e3-97bf-b9a256684eaf.png)