Message.hs: Replace "theadID" with "threadID"

Fixes #23.
Remove IsString instance of `Jid'

IsString instances are never supposed to fail, but parsing Strings to
JIDs might. We might do this later with Template Haskell instead, or
something.
README.md: Clarifications, mention ICU dependency, and conform to API
pontarius-xmpp.cabal: Add bugtracker URL and change descriptions
examples/echoclient/Main.hs: Adapt to new API
Merge pull request #29 from jonkri/master

Apply various minor fixes