fix bug: prefetch causing problem when user move mouse fast above links
queuing a lot of fetch commands which causes cache to throws errors

how to repeat
create bunch on links near each other, like lists of search results with link one above other
each link must be returned with real world latency - 200 ms
move mouse fast above these links

observe:
browser display loading icon below cursor
watch console and see errors
Merge branch 'master' into prefetch-bug

Conflicts:
	jquery.smoothState.js
when prefetch idle, fetch queuedPrefetchUrl
when prefetch idle, fetch queuedPrefetchUrl - fix jsdoc
Merge remote-tracking branch 'upstream/master' into prefetch-bug

Conflicts:
	jquery.smoothState.js
prefetch - fix bug with queueUrl blocking prefetch
prefetch - prevent clear cache throwing error
prefetch - clearer names / jsdoc
Merge pull request #18 from LPodolski/prefetch-bug

fix bug: prefetch causing problem when user move mouse fast above links