Json: properly decode utf8

actual UTF8 encoded data caused the JSON store to die. Proper invocation
of JSON methods fixed this.
properly print non-ascii contents on utf8 terminals

not sure if I got this right, but perl seems to default STDIN/OUT/ERR to
ascii/latin1/non-utf8.

So, if you print some non-ascii data with utf-8 flag on, the output is
"unexpected".

This sets the binmode according to LANG/LC_CTYPE...
Json: fix power with recint != 1sec

power was completely wrong for recints != 1sec. fixed.
Store: skip writing empty files

... at least until significant global data is handled.