added get-node function to get all existing values in a node position

I've added a new function usefull to get a list of existing values under a selected node position given a metric pattern, i json format

__parameters:__
* query: metric pattern 
* node: portion/s of the target name. Node indices are 0 indexed.

example:

http://graphite_url/metrics/get-node/?query=*.bbdds.sys*.system.*&node=2

give us

{"nodes": [{"name": "system00"}, {"name": "system01"}]}

this will be useful for external dashboards  to build dinamically combo boxes  to select a substring for a complete metric name.