Merge Infinite Scroll branch as of r619593 into trunk.

The trunk to branch merge in r617814 caused some issues, I believe, resulting in this merge trying to remove `modules/infinite-scroll.php` and `modules/infinite-scroll/`; the problem was dealt with manually.
Infinite Scroll enhancement

Introduce `pushState` tracking for user position in standard WP archive pages. In conjunction with jQuery Sonar, will reduce brokenness of back button during IS interactions.
Infinite Scroll enhancement

Introduce methods for updating URLs via `pushState()` that take into account users scrolling up and down a page, rather than simply down a page.

As these methods are still undergoing development and testing, they are disabled for the moment. Once refined, they will be incorporated into the Scroller method and enabled. Hopefully these will be ready for the 2.0 launch.
Enable Infinite Scroll URL handling for 2.0 beta 3.
Remove unnecessary URL update call in IS code that causes URL to change, then be changed back by code introduced in r619972.
Incorporate Infinite Scroll archive URL code from r619972 into main `Scroller` functionality. Represents relocation, rather than major refactoring, of code from said revision.
Infinite Scroll URL tracking update

Don't bother if IE version is less than 10, since `pushState()` isn't supported.
Update bundled Infinite Scroll support for default themes.

Specifically, in Twenty Twelve, check for sidebar widgets on mobile devices since sidebars move below post list.
Update Infinite Scroll URL determination to accomodate IS containers with floated children.

Since `jQuery.height()` returns `0` when children are floated, the code cannot determine which set is in view. In this situation, we sum the heights of the floated children to determine the container's height.

Also, we switched to `jQuery.outerHeight()` to ensure that a container's margins are included when its height is computed.
Remove WP.com-specific handling for mapped domains when determining Ajax URL for Infinite Scroll requests
Add textdomain
Remove Jetpack-specific elements from main Infinite Scroll file, introducing a class for these items in the module file. By doing so, the main IS files in Jetpack and on WordPress.com can be kept in sync.
Remove erroneous PHPDoc related to code moved in r622288.
Provide a filter on the Infinite Scroll footer credits.
Jetpack: Infinite Scroll: don't force a border as the default style of infinite-wrapper.
props matias
When Infinite Scroll module checks for bundled support, also check the parent theme if the active theme is a child.
Revert r622735 for the main Infinite Scroll file. Functionality will be moved to the Jetpack helper, since it is only needed in the plugin.
When gathering additional assets to load, check if source contains full path and handle accordingly.
Ensure that main query's arguments are respected when Infinite Scroll queries are executed. See http://wordpress.org/support/topic/infinite-scroll-category-archives-not-working-properly.
Infinite Scroll JS update

Switch back to `.on()` and `.off()` to resolve various problems with `.live()`.