"Twitter's Bootstrap" => "Bootstrap" & update Bootstrap link

Per http://getbootstrap.com/about/#brand
Merge pull request #3375 from cvrebert/master

"Twitter's Bootstrap" => "Bootstrap" & update Bootstrap link
Slight copy adjustments for the last merge.
Update README to reflect that issue tracker is disabled on this repo
Merge pull request #3376 from cvrebert/patch-2

Update README to reflect that issue tracker is disabled on this repo
Merge branch 'master' of https://github.com/blueimp/jQuery-File-Upload