New Java API: creating Encoder

**Issue by [ktoso](https://github.com/ktoso)**
_Thursday Feb 25, 2016 at 17:01 GMT_
_Originally opened as https://github.com/akka/akka/issues/19882_

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The old JavaDSL does not allow creating Encoder instances in javadsl.
The new one will be able to, we just need to add it ;-)

Start by locating the new JavaDSL API and the existing Encoder creation paths, then compare them with the old JavaDSL. The work is complete when the new javadsl exposes a way to create Encoder instances; the issue does not name specific files or tests.

Written by the indexing model from the issue text.