Add some `Jid' utility functions; don't allow construction of `Jid'

Fixes #32.
Merge pull request #33 from jonkri/master

Add some `Jid' utility functions; don't allow construction of `Jid'
Merge remote-tracking branch 'upstream/master' into upstream
remove to prefix from toLocalpart, toDomainpart and toResourcePart
quell ghc warnings
check for roster versioning stream feature and disable roster versioning unless present