Handle Unicode autocomplete.

A patch to go.crypto/ssh/terminal[1] added support for Unicode input and
changed the autocomplete API accordingly. This change updates
xmpp-client to use the new API.

With this, inputting Unicode characters should mostly work, except that
grapheme clusters are know to have problems.

[1] https://code.google.com/p/go/source/detail?r=ff9ce887b46be1f2c2e72ebe3a7ee3f8645a63b6&repo=crypto

Fixes #39