enable specification of the root metric path at send time.

when sending multiple hashes, don't require the path attribute to be specified when instantiating the Net::Graphite object; instead support adding it as a parameter when calling ->send().

passing in a hashref of metrics without specifying any path results in lots of warnings such as: 'Use of uninitialized value $path in concatenation (.) or string at /usr/local/booking-perl/5.14.2/site/lib/Net/Graphite.pm line 113.', and the resulting metrics have an empty '.' prefix (luckily, Graphite ignores the empty prefix... FOR NOW)