More descriptive error message when json_pack fails

It's not exactly helpful now but check_mem is just wrong here
Updated README

If you use ttrss-mode "single" it always uses "admin" as the username...
Add new JSON encoder

jansson was throwing a fit when I tried to run json_pack on strings that
weren't UTF-8 (eg article content that uses fancy quotes) and after
examining alternative libraries (which are all completely horrible) I
decided to write a JSON encoder myself. It has a nice Maybe-monadic
interface so you can basically ignore errors and it doesn't `exit` when
you run out of memory like a certain library I looked at does.