Add configurable graphite path. Fixes issue 87

Deprecate the "graphite_prefix" attribute in favor of "graphite_path"
which is configurable via macros (%s %m %h). Sane defaults ensure that
existing configurations are not broken by this change. This should solve
issue 87 and any future needs in its general direction. I've moved code
relating to graphite and memcached from rrdhelpers.(c|h), to
export_helpers. This is housekeeping that reflects how those features
should have been implemented in my original graphite patch.