Add back-end protocol 3 support

[ai]

Version 3 of back-end protocol bring changes:

1. password → secret
2. ["error", meta.id, stack] must contain action ID.
3. An error outside of action processing (where we do not know meta.id) the server should answer 500 HTTP code

[anaumov]

@ai let's jump into protocol v4?

[ai]

Yeap. Having Protocol 4 will be the best. You can start adding 4 to PR for 3.

[anaumov]

@ai I'll create new one. Let's release gem with protocol 3. What do you think?

[ai]

@anaumov @dreikanter yeap, it is better to release gem for 3 and then release for 4.

[dreikanter]

@anaumov @ai I will release V3 support as a separate gem version on RubyGems later this week. Since #11 is already merged to master, it would be nice to have V4 support implementation on a separate branch to simplify further merge process.

BTW, do we have breaking changes between V3 and V4 protocols?

[ai]

Yeap V4 is not compatible with V3