Bosh monitor graphite

This change introduces a graphite plug-in for bosh-monitor. It also includes a refactoring of existing tsdb plugin as graphite plug-in works in a similar way. Refactoring consists of extraction of parent class tcp_connection which contains the common code used by tsdb_connection and graphite_connection.
There is also a new optional  parameter introduced in the graphite plug-in config named 'prefix' which allows
users to optionally specify prefix in their graphite metric path.