Suggested solution for #8

As I think the `SaslHandler's might change relatively frequently
depending on the value of `ConnectionState', I hereby suggest that the
handler value passed to `session' is changed from `[SaslHandler]' to
`(ConnectionState -> [SaslHandler]'.

Most people would probably just do `\_ -> scramSha1...', but I guess
the extra flexibility this provides makes up for the slightly more
complicated API.
Merge branch 'master' of github.com:pontarius/pontarius-xmpp