Add timezone option for users

[sethwoodworth]

• add timezone to models.UserProfile
• default timezones to EDT/EST or the server timezone setting
• import TZ to templates that render datetimes {% load TZ %}
• add location to set user timezone in edit_profile (when editing profile exists #210)
• wrap template date display tags with {% timezone user.get_profile.timezone %}…{% endtimezone %}

https://docs.djangoproject.com/en/dev/topics/i18n/timezones/#time-zone-aware-output-in-templates