Updated search and spinner patterns.

# Changes
 - Added `.loading` modifier to `input[type="search"]` which shows a spinner in place of the search icon.
 - Button's `.loading` modifier class now works with `input[type="submit"]` (because it is no longer relying on pseudo-elements).
 - Using SVG for search and spinner graphics so they are resolution independent. :tada: Includes GIF and PNG fallbacks for older browsers.

Tested in IE8, IE9, IE10, IE11, Chrome (latest), Safari (latest), Firefox (latest).
For review: @DoSomething/front-end 

# Screenshot
__Search with loading class:__
![search loading class](http://f.cl.ly/items/3N1J2P2S2R2w3l0T0i0l/Screen%20Recording%202014-10-07%20at%2004.27%20PM.gif)

__Button with loading class:__
![button loading class](http://f.cl.ly/items/0O332m3G0Q0x1m3F003U/Screen%20Recording%202014-10-07%20at%2004.31%20PM.gif)
Modernizr ftw!