Update App.Transitions' API docs to be more consistent.
Add `Y.App.Content`, a `Y.App` class extension.

This extension provides pjax-style content fetching and handling. This
makes it easy to fetch server rendered content for URLs using Ajax. The
HTML content returned from the server will be view-ified and set as the
app's main content, making it seamless to use a mixture of server and
client rendered views.

Closes #2532487
Add unit tests for App.Content.
Minor tweaking of Pjax's API docs.
Build Pjax.
Build App.Base, App.Content, App.Transitions, and Router.
Build Loader for App.Content.
Merge branch 'app-content' into 3.x
Optimized facade mixing a little more.

Ideally it seems like we could drop hasOwnProperty here, since
we have a whitelist, but if(o in p) without the hasOwnProperty is
just as slow in Chrome 21 as the current implementation, if not slower
(which kind of makes sense to me - I would expect if(o.hasOwnProperty(p))
to be faster than if(p in o) in theory - would be surprised if that optimization
shows up in other engines.

So, anyway, going with this approach which is the only one which improves on the
current implementation across all targeted browsers.

http://jsperf.com/hop-in/12

NOTE: I saw the same results in native Chrome profiling also.
Build for event facade further optimizations
Merge commit 'upstream/3.x' into event-perf
Extended builder logger regex for event-custom-* builds to remove this.log/self.log calls

CE supports a "silent" flag, which allows users to optionally disable log messages in debug
files for as specific type of event. The CustomEvent.prototype.log method was used to
encapsulate the logic for this flag, and hence CustomEvent code used this.log instead of
Y.log directly.

These calls were being left in the min files and resulted in function hop overhead, which
at CEs scale, bubbled up in profiles. NOTE: We never actually logged anything in the min
files, since the Y.log call was stripped. It's just the unneccesary function hop we're trying
to reduce here.
Build for this.log/self.log stripping
Added transformOrigin and msTransform to dom-style.
Improve performance of Y.Attribute#getAttrs()

There was an unnecessary extra attribute retrieval per attribute, which
caused a major performance hit. This change removes the unnecessary
retrieval and adds a few other micro-optimizations.
Close #2532673: Remove X-Requested-With header with XDR
Build IO Base
Optimized _monitor, to accept a custom-event, and also assume a prefixed type to cut down on Y.cached calls.
Build for optimized _monitor.

Still need to try and avoid _monitor function hop where possible,
without re-invoking getEvent.
Tweaked App.Content unit tests.