Add support for Jinja2

- templates are searched in indico/web/templates/
- bytestrings are converted to unicode automatically EXCEPT for
  {% trans %} arguments - use |ensure_unicode manually there
- additional functions available globally: url_for (with indico extras),
  url_rule_to_js, formatDateTime, formatDate, formatTime
Use new date/time format functions in Jinja

Also ensure that the result of a custom filters is unicode
Add function to access jinja macros from python

e.g. get_template_module('foo.txt').some_macro()
Separate module for jinja stuff, underline filter