added a check to ensure we have a path for the data files to be housed in when adding new metrics.  this can otherwise fail

my metrics weren't showing up, and log files growing -- whipser was trying to put a `.wsp` file in a directory that didn't exist.

simple fix -- use `os.makedirs` to create the intermediate directories

others seem to have had this issue as well.  comments in https://github.com/graphite-project/carbon/issues/11 and a few other places.

the same 3 lines works in the 0.9.10 release as well  -- just outdented 2 spaces and put on line 327