Add updated Brackets icon

[caazdev]

Added updated Brackets icon to better follow the Human Interface Guidelines.

The previous icons, though did resemble elementary OS's style, didn't completely follow the guidelines they're based on. I did my best to fix that (in all six sizes).

Above is a clear comparison between the original and the modified version (64x64), and here's a screenshot of the slingshot + plank integration.

[quassy]

Thanks for your pull request which is a great first contribution! Your new icon looks much better indeed, but it'd be great if you could fix two problems I came across, so I can add them to elementary+.

The positioning of the 128px icon is a bit off as the baseline is too high when I compare your icon with for example the Terminal icon on the elementary HIG but also our current Foxit Reader icon.

Also the 32px icon looks slightly too big: Terminal, Foxit Reader.