Texts in sieve must be in UTF-8 charset

When writing texts in sieve, the sievec compiler
requires them to use the UTF-8 character set.

Here is a patch that makes websieve.pl use utf-8 in all
communication with the web browser, and hence websieve
will get all texts in utf-8 and pass this on to
timsieved and hance sievec.

I suggest this patch gets comitted to the distribution.
With the current released websieve, there is an error
and the script cannot be saved if I use eight bit
characters.