REST API discussion

This ticket is for discussing adding REST API support.

How EOS is doing REST API is mentioned in https://github.com/bitshares/bitshares-core/issues/792#issuecomment-383532647 and https://github.com/bitshares/bitshares-core/issues/792#issuecomment-383419223 by @Zapata.

> Note that EOS also have moved from JSON-RPC to REST based API (see [doc](https://eosio.github.io/eos/group__eosiorpc.html#configuration)).

> Exposing Bitshares in a REST API style might be nice in the long term, but I don't think it worth the effort for the "API Documentation" subject. It will require to expose both style for a smooth migration as all the clients need to be updated.
For reference EOS has a [htttp_plugin](https://github.com/EOSIO/eos/blob/master/plugins/http_plugin/http_plugin.cpp), wich is used in [this way](https://github.com/EOSIO/eos/blob/master/plugins/account_history_api_plugin/account_history_api_plugin.cpp#L49).

Here is another project about REST API: https://github.com/bitshares/Bitshares-HUG-REST-API