Added current(last) value to timeseries graph

- added new option @show_current to timeseries graph

@show_current: true to show current/last value on graph legend

Refactored by_type function current to use d.last that we store in
process_data since we already pull current/last value there.
Pull ymin from graphite and use it in discover()
Merge pull request #49 from Crossway/master

Pull yMin from graphite and use in discover()
Merge pull request #48 from drspin/feature/timeseries_current_value

Feature/timeseries current value