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.
Change tabs to spaces
Restrict -R usage to when -B (and -b) are set.
Update rrdcached documentation with new switch and its usage.
Merge pull request #414 from Zsub/feature/rrdcached-create-recursive

Add an option to let rrdcached recursively create directories.