Add contentParserCallback to d3.xhr constructor

Fixes race condition that exists where the xhr.onload fires before
xhr.response has had a change to be set. This causes d3_json for example
to not get called.
Update to Mike's approach
fix dumb merge issues
supposed to call private d3_xhr
If xhr.response errors, callback with error.

Fixes #1263. Previously, if d3.json was used to load invalid JSON, a SyntaxError
would be thrown. Now an error is reported to the callback instead, in the same
manner as if a network error occurred. This allows the caller to handle the
invalid response, rather than throwing an asynchronous error.
Fix dangling global in projection.clipExtent.
Custom UglifyJS script to warn about leaking globals.
fix simple code review issues
make diff nicer
Fix for method overloading in d3.xml and d3.text
Insert new tasks on the end.

This is a partial fix to #1245 whereby timer tasks are now invoked in the order
in which they were registered, rather than in the reverse order. However, a
better fix remains, which is to invoke tasks in the order of their start time
rather than their registration time.
Fix assumed tween order for transition tests.
Simplify timer data structure.

Rather than store the reference time (then) and a delay, timer tasks are now
stored with only a start time. A task becomes active once the current time
passes the start time. This includes a small change to the behavior of flush,
such that any active timer task is triggered, rather than only triggering tasks
with a delay of zero. The resulting behavior is for most purposes equivalent.
Remove comment now that order is defined.
Minor optimization.
Delete defunct rescheduling of timers.

In a much earlier time (e25acce12489ea8fa3ed9e09bdc3b7cb5d6d7f8e), transition
delays were implemented by rescheduling the step callback with a new delay;
thus, d3_timer had to support finding an already-registered timer callback and
changing its delay. This was not supposed to be part of the public API when
d3.timer was later exposed, and is no longer needed due to the Great Transition
Rewrite of D3 3.0. So, we can simplify and optimize the code by deleting this
defunct feature.
Fix global.
Rename for readability.
Merge branch 'fix-xhr-race-condition' of github.com:mrcarlosrendon/d3 into fix-xhr-race-condition
Add generated files.