Add list_series and delete_series methods to Client

[wrocla]

It would be nice to have list_series and delete_series methods on Client,
like list_databases or delete_database.

e.g.:

influxdb = InfluxDB::Client.new ENV['INFLUXDB_DATABASE'], ['INFLUXDB_USERNAME'], ['INFLUXDB_PASSWORD']
influxdb.list_series
influxdb.delete_series(series_name)

I made PR in two versions.
#184

• add execute_db to Query::Core

#185

• modify execute method in Query::Core

[dmke]

Cool!

I'll have a look later, when I'm back home (was just about to get to work).

[dmke]

I'm in favour of #185 (i.e. I don't like the additional execute_db method).

I have some comments though, which I'm going to add over there.