Add an option to let rrdcached recursively create directories.

If a `create` command is given, a path is accepted. This option enables rrdcached to create the directory structure if it does not exist yet.

The reasoning for not implementing the same functionality in rrdcreate is that a clean implementation would require a new function with a new function signature, and I am not feeling quite confident enough to change that :) Furthermore, when running rrdcreate locally (without a daemon) supposedly one will be able to create directories on that machine.

Anxiously awaiting your comments!