Fix the detection of the 24 hours time format

Moment.js allows to escape characters by enclosing them in brackets. The expression used to determine whether to use the 24 hour format wasn't able to distinguish between escaped and unescaped characters. By using a regular expression it's possible to match the characters "h" and "a" (that signal to use the 12 hour format) only outside of brackets.
Update bootstrap-datetimepicker.js
Merge pull request #600 from wynksaiddestroy/fix-24h-format-detection

Fix detection of 24 hour format