Enable configuration of custom logic for transformation of map keys

[sirthias]

Currently there is no way to customize the handling of map keys during decoding, which means that the names have to line up perfectly. If it was possible to hook in custom logic like _.toLowerCase oder _.toCamelCase flexibility for decoding serializations from 3rd party sources would be greatly increased.

Also, it'd be great to have an annotation allowing for setting alternate names of case class members in serializations.

[OndrejSpanel]

The encoding might be also able to handle non-string types, therefore implementing most of #315