split message text by word boundaries

If the message text needs to be split into several SMS, try to split it
by word boundaries rather than by characters only.

Fixes: https://code.google.com/p/esmska/issues/detail?id=110
Contributed by: Pavel Zak <zakpave3@fel.cvut.cz>
Pull request: https://github.com/kparal/esmska/pull/13