Reworked Producer Configuration, added ThreadingModel support

PR: #30

See below for the new producer instantiation, you now use ProducerConf for the Scala implementation too.

New method:

val kinesisConfig: Config = ConfigFactory.load().getConfig("kinesis")
val kpl = KinesisProducer(ProducerConf(kinesisConfig, "some-producer"))

Note that the config is the kinesis block, not the kpl block as before.

Old method:

val kinesisConfig: Config = ConfigFactory.load().getConfig("kinesis")
val producerConfig: Config = kinesisConfig.getConfig("some-producer")
val streamName: String = producerConfig.getString("stream-name")

val kpl = KinesisProducerKPL(kinesisConfig.getConfig("kpl"), streamName)

• Small breaking change Removed the trait in the Producer and renamed KinesisProducerKPL to KinesisProducer
• Added ThreadPoolSize and ThreadingModel KPL properties, see awslabs/amazon-kinesis-producer#100
• Refactored how the properties are loaded to share commonality between the actor and Scala
  implementations. This simplifies the Scala implementation and makes it more robust. It also allows more control over the underlying KPL properties should someone want to not use the Typesafe config approach.