Lazy create and memoize InfluxDB::Rails.client

Instantiating an async InfluxDB::Client creates thread workers. With the basic
setup suggested in the README, we call the InfluxDB::Rails.configure block
twice which creates 2 client instances (which each start InfluxDB::Workers)
Merge pull request #15 from chingor13/lazy_load_client

Lazy create and memoize InfluxDB::Rails.client