bson: rename bson_append_string() to bson_append_utf8().
bson: add missing headers to INST_H_FILES.
bson: remove libunistring for utf8 validation.

This uses the utf8 validation used in the python mongo driver. It's
pretty rudimentary but should get the job done. It also reduces our
external dependencies to just libc.
cbson: fix cbson.DBRef.collection getter.
cbson: make sure we return None for empty database.