Change naming conventions.

Instead of poem_create(), poem_name() etc, use the more natural
create_poem(), get_poem_name() etc. Also instead of verse_poem()
which does not really make sence, simply use get_poem() instead.
As long as there's no clashes in namespace this should be clear
enough.
Add xml formatter.
Fix off by one error in strbuffer.c.

When appending we need to make sure we have room for
string zero terminator too.