Visually centered a few icons, some new paths and optimizations. (#75)

* Visually centered a few icons, some new paths and optimizations.

Apple - Visually centered and change the path to more a accurate one.
Bytes: 1011 —> 580.

Dropbox - Visually centered and change the path to fix point alignment.
Bytes: 338 —> 350.

Twitter - Visually centered.
Bytes: 425 —> 431.

Ubuntu - Visually centered and change the path and color to a more
accurate one.
Bytes: 755 —> 674.

Vimeo - Visually centered.
Bytes: 375 —> 379.

WhatsApp - Visually centered and change the path and color to more a
accurate one.
Bytes: 646 —> 611.

Windows - Visually centered and change the path to fix the lines
between shapes.
Bytes: 274 —> 284.

Hope this helps.

FYI: Wikicommons have good SVG shapes and most companies have brand
guidelines with accurate logo renditions and colors.

* Removed newlines, fixed Ubuntu color and updated ReadMe and Twitter PNG.

images/svg/dropbox.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" aria-label="Dropbox" role="img" viewBox="0 0 512 512"><rect width="512" height="512" fill="#fff" rx="15%"/><path fill="#007ee5" d="M256 161l-112 72 112 71-112 72-113-72 113-72-113-71 113-72zM143 399l113-72 112 72-112 72zm113-95l113-72-113-71 112-72 113 72-113 72 113 71-113 72z" class="st0"/></svg>
+<svg viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg" aria-label="Dropbox" role="img"><rect fill="#fff" height="512" rx="15%" width="512"/><path d="m256 142-112 71 112 71-112 72-113-72 113-71-113-71 113-72zm-113 237 113-72 112 72-112 72zm113-95 112-71-112-71 112-72 113 72-113 71 113 71-113 72z" fill="#007ee5"/></svg>