Use ugettext_lazy instead of ugettext to make it compatible with Django 1.7

https://docs.djangoproject.com/en/dev/ref/applications/#applications-troubleshooting
Use json (from stdlib) instead of simplejson from django utils
Add 'jsmin' to the requirements
Add missing underscore-min.js dependency on 'bootstrap_calendar_js' templatetag
Updated to master version of bootstrap-calendar

https://github.com/Serhioromano/bootstrap-calendar/commit/20e1a2aef1def5ba25864361e4496f1450ecbcb7
CalendarEvent model changes

 - make "url" not required: an event can have not url at all

 - use "choices" keyword argument for "css_class" so it is easier to
   select it from the admin site
Add 'width' attribute to the template tag
Merge pull request #13 from humitos/master

Make it work on Django 1.7