graphite datasource api to support multiple backends as plugins

[Dieterbe]

make it easy to add new backends ("timeseries metrics providers") to graphite without hacking graphite's sources, i.e. some kind of plugin mechanism.

• @jordansissel has shown interest for doing this with logstash (https://gist.github.com/jordansissel/3760225)
• chris larsen (not attending monitorama) has shown interest for doing this for opentsdb: https://answers.launchpad.net/graphite/+question/223956
• interesting distributed metrics database that could also be a good fit: https://bitbucket.org/0x0000/saturnalia/wiki/Home (not sure how this compares to ceres/megacarbon, though)