Release 0.2.0 (Implicit Serdes, more typesafety, misc refactorings)

Here are the most important highlights of this release:

1. Implicit Serdes: All serializers and de-serializers are now passed as implicit serdes. APIs that take instances of Serialized, Consumed, Produced or Joined work with implicit serdes in scope. Please take a look at README for details.
2. Added type-safety: Removed support for default serdes through config. This makes the library more compile time type-safe, as any missing serdes will now be flagged as a compile time error. README has the details.

The following issues have been fixed in this release:

1. Implicit Serdes
2. KTable#filter(K,V => Boolean, Materialized) is called recusively on KTableS
3. Kafka server utils not being packaged
4. java.lang.ClassCastException for reduce api call