Added support for utf-8 upper case letters.

javaScript does not seem to support unicode upper case/title case character
classes, so this has to do.
Cleaned up some code samples.

I found these as I was correcting accidental duplicate 'var' occurences.
Allowed date to read YYYY/MM[/DD].

It also allowes YYYY.MM[.DD], who cares??
Implemented experimental JavaScript eval filter.

This needs a lot of work because it is both unsafe and unusable!!!
- Fields have to be prefixed with 'doc' every time and I cannot imagine that
  anyone really wants to do this.
- Variables can be overwritten at will.
- _list/tabulate will cause a 500 if syntax is wrong.
On a lighter note, it should be easy to provide sum(), min(), max() functions,
so this has the potential to cover simple SQL expressions etc.
Re-organized _list/tabulate.

Internal content_matches() was getting less and less atomic and hence was split
into four parts of ascending complexity: defines_field(), matches_value(),
contains_keyword(), matches_expression(). They explain themselves sufficiently.
Improved security in _list/tabulate's expression evaluation.

Namely I protected every variable/function previously in use and a catche error
also results in a skipped row.
Simplified _list/tabulate API and map.html.

Reverting the state to the form just before the new filter was issued went more
and more complicated and also the use of an <option value="_expression"> was
confusing. I moved cut.expression to cut.value where it made more sense, and
introduced special cut.field identifiers '_expression' and '_keyword'. Those
coincide with my option values on the map page.
I'd still say that '_keyword' is a bit misleading; it is just an ordinary
RegExp that is applied to the full document. Same goes for '_expression': it
does not imply "JavaScript expression" and may easily be confused with "Regular
Expression" or something.
Replaced some ternary operations with bools.
Simplified expression syntax.

It truly looks like RegExp nightmare and manages to transform this:

'main field from my document' . sub_field == "some nasty string we've got here gov'na!!"

into this:

doc["main field from my document"]["sub_field"] == "some nasty string we've got
here gov'na!!"

I.e. "string literals" are preserved (even with single quotation marks) and
'variable names' are interpreted as doc members. Unfortunately the \w pattern
does not include äöß etc. so these had to be added explicitely. Also, I should
add a couple of comments because this one RegExp is overwhelmingly long.
Rephrased _expression option label.

Since it isn't JavaScript anymore, I exchanged the label with something I also
don't like. It reminds me of Excel.