generalize to support more than one sink type backend

support for influxdb backend is on its way
free the memory buffer when we're done with it
factor out timer stats computation in a dedicated function

_compute_timer_stats
add support for influxdb backend
update optionparser doc for influxdb backend
add support for proxy_values custom type

this will proxy all received values instead of aggregating these into one
unique value
(influxdb support only for now)
add support for proxy_values custom type for graphite backend
time precision is different for graphite vs influxdb
add support for proxy value to simple client lib
Merge branch 'proxy-value-type'
update .gitignore
bump version to 0.7
update debian/changelog