adding override config directory

Adds option to override default config files with user defined config files for graphite.

--config <dir> with custom configuration files.  Supports following files:
carbon.conf
local_settings.py
storage-aggregation.conf - this file was not setup in original version.
storage-schemas.conf

config directory with user defined files are used as overrides meaning if this directory doesn't contain a config file, a default will be used.  also files out of this directory are copied rather than moved.