Changed <select> to use actual values and not strings + some bugfixes

- <select> with static elements now still uses angular machinery, this
allows to pass values as values and not strings (might be improved more
later for groupby, labels etc)
- fixed a bug in attr_if
- updated example.py
- <select> placeholder is now a $text and not a $bool
- added global debug() function to jinja2 environment
- added $string and $number value types (ui.select options list
currently only accepts those)