Custom header types, scalameta escaping bugfix

Included issues:

• #88 Adds support for other encoding charsets for JSON payloads
• #88 Using a plain unmarshaller for text/plain
• #88 Removing jsonFSU from clients in favor of the ones in AkkaImplicits
• #88 Returning NoContextException from failed unmarshallers instead of the Circe exceptions so we can try the next available unmarshaller

Contributors:

• @blast-hardcheese