WIP - Refactoring for release 0.1

[wzorgdrager]

Related issue(s): #163

Description of changes

This PR covers some refactoring for release 0.1.

Refactoring changelog

• Rename PipelineObject to Stage. This name is more intuitive given we are using (Input|Transform|Output)Stages. Consequently, PipelineObjectList is also renamed to StageList.
• Added appendSource(StreamExecutionEnvironment => DataStream[T]) to create an anonymous InputStage (append already exists for both Transform and Output stages).
• PipelineBuilder allows for setting the StreamTimeCharacteristic instead of always setting it to EventTime.
• Custom serializer functionality (see Make Serializers external #163)
• Added ScalaDoc to all core classes.
• Minor bugfixes to all core classes.
• Minor renamings/refactorings to all core classes`.

Check all classes and tests

• Correct Scaladoc
• Correct formatting and style

Core

• Review classes and tests
• Fix KafkaClient bug in tests (connection time-out)

Check-list

Please make sure to review and check all of these items:

Core

Buffer

• Buffer
• BufferFactory
• BufferType
• Exceptions
• KafkaBuffer
• RabbitMQBuffer

Serialization

• AbstractSerde
• BsonSerde
• JSONSerde
• KryoSerde
• Serializer

Schema Exposure

• SchemaExposer
• ZookeeperSchemaExposer
• RedisSchemaExposer

Keymanager

• KeyManager
• StaticKeyManager

Redis

• RedisKeyManager

Pipeline

• DirectedAcyclicGraph
• Exceptions
• Pipeline
• PipelineBuilder
• PipelineObject -> Stageutilties
• PipelineObjectList -> StageList
• PipelineObjectN -> StageN
• Types

Stages

• InputStage
• OutputStage
• TransformStage
• StageAttributes

Kafka

• KafkaInput
• KafkaOutput

RabbitMQ

• RabbitMQInput
• RabbitMQOutput

Utilities

• DuplicateService
• FiniteQueue
• HttpRequester
• JsonPrinterOutput
• PrinterOutput
• SeqInput
• StringInput
• Properties

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

[wzorgdrager]

Instead of creating this huge PR, I will merge and close this one soon (mostly refactoring). To split the other work into smaller PR's. That will be way more clear.