Compress png images further using pngtastic + zopfli compression

We can crunch the png images a bit more, saving precious bytes!

See my image compressor, and google's new deflate algorithm it relies on here:
https://github.com/depsypher/pngtastic
https://code.google.com/p/zopfli/

Here is the relevant output. Only the files that have size savings are included in the PR.
/img
 7.01% : 12799B -> 11902B (  897B saved) - ./glyphicons-halflings.png
 0.00% :  8777B ->  8777B (    0B saved) - ./glyphicons-halflings-white.png

/docs/assets/img
 0.00% : 30612B -> 30612B (    0B saved) - ./bootstrap-docs-readme.png
10.14% : 11244B -> 10104B ( 1140B saved) - ./bs-docs-bootstrap-features.png
 0.00% :  6450B ->  6450B (    0B saved) - ./bs-docs-masthead-pattern.png
20.82% : 10572B ->  8371B ( 2201B saved) - ./bs-docs-responsive-illustrations.png
 5.47% : 30968B -> 29274B ( 1694B saved) - ./bs-docs-twitter-github.png
 0.00% :  8777B ->  8777B (    0B saved) - ./glyphicons-halflings-white.png
 7.01% : 12799B -> 11902B (  897B saved) - ./glyphicons-halflings.png
 8.33% :    84B ->    77B (    7B saved) - ./grid-baseline-20px.png
 0.58% : 12824B -> 12750B (   74B saved) - ./less-logo-large.png
 0.00% :  1008B ->  1008B (    0B saved) - ./responsive-illustrations.png

/docs/assets/img/examples
 1.27% :339980B ->335663B ( 4317B saved) - ./bootstrap-example-carousel.png
 0.13% :209039B ->208763B (  276B saved) - ./bootstrap-example-fluid.png
 0.00% :136021B ->136021B (    0B saved) - ./bootstrap-example-justified-nav.png
 0.00% :117303B ->117303B (    0B saved) - ./bootstrap-example-marketing-narrow.png
 0.00% :134269B ->134269B (    0B saved) - ./bootstrap-example-marketing.png
 0.58% : 22037B -> 21910B (  127B saved) - ./bootstrap-example-signin.png
 3.37% : 36099B -> 34883B ( 1216B saved) - ./bootstrap-example-starter.png
 2.90% : 30820B -> 29927B (  893B saved) - ./bootstrap-example-sticky-footer.png
 1.78% : 55522B -> 54535B (  987B saved) - ./browser-icon-chrome.png
 0.08% :175994B ->175849B (  145B saved) - ./browser-icon-firefox.png
 0.33% :209527B ->208830B (  697B saved) - ./browser-icon-safari.png

/docs/assets/img/example-sites
 2.53% : 62853B -> 61265B ( 1588B saved) - ./8020select.png
 2.72% :136480B ->132768B ( 3712B saved) - ./adoptahydrant.png
10.38% : 72725B -> 65179B ( 7546B saved) - ./breakingnews.png
 0.59% : 39837B -> 39603B (  234B saved) - ./fleetio.png
 6.16% : 76560B -> 71844B ( 4716B saved) - ./gathercontent.png
 1.75% :  7258B ->  7131B (  127B saved) - ./jshint.png
11.01% : 48259B -> 42947B ( 5312B saved) - ./kippt.png
 1.07% : 50225B -> 49686B (  539B saved) - ./soundready.png

/docs/assets/ico
 0.94% : 11392B -> 11285B (  107B saved) - ./apple-touch-icon-114-precomposed.png
 1.51% : 16780B -> 16526B (  254B saved) - ./apple-touch-icon-144-precomposed.png
 1.44% :  4026B ->  3968B (   58B saved) - ./apple-touch-icon-57-precomposed.png
 0.32% :  5681B ->  5663B (   18B saved) - ./apple-touch-icon-72-precomposed.png
39.54% :  2711B ->  1639B ( 1072B saved) - ./favicon.png

Thanks for looking :)