Datepicker: Define en and en-US locales based on defaults

Fixes #6682
Closes gh-1269
Build: Reorganize external directory

Put each external library into its own directory.
Move jquery.js to external.

Ref gh-1266
Test: Get all jQuery versions from Bower

Ref gh-1266
Build: Upgrade to grunt-bowercopy 1.1.0

Ref gh-1266
Bower: Change jquery dependency to >=1.6 (don't limit max version)

Fixes #10110
Closes gh-1266
Merge branch 'master' of git://github.com/jquery/jquery-ui into 7953-datepicker-show-year-month-in-different-lines