Support maps, deserialize to maps instead of HashDict

In order to get a little experience with maps I've made a patch that makes elixir-json use maps as the primary translation of objects. HashDicts are still accepted as inputs.

This is based on the v0.13 branch of Elixir (which requires Erlang R17 to be installed) and the parser-decoder-decoupling branch of elixir-json as it seemed that's where most of the development effort is being done.

I hope this will be useful.