prevent the segment xhr error handler from being called during seek

When an XHR is aborted, the error handler is called. The error handler for the
segment loader increments the mediaIndex of the player. This is undesired
because setCurrentTime has already set the mediaIndex to the proper target
segment.
add a cancelKeyXhr akin to cancelSegmentXhr
Merge pull request #177 from videojs/cancelXhrs

Cancel xhrs