Make blockchain_explorer part of exonum crate

[defuz]

• Binary exonumctl seems like a natural part of exonum crate.
• Common public API definitions too.
• We may live with helpers.rs into exonum crate.

@alekseysidorov @gisochre Any thought?

[dj8yfo]

This looks like a natural first step to #66.

@alekseysidorov What should each Service return? BTreeMap<"endpoint_suffix", H> where H:iron::Handler ?

[dj8yfo]

So these modules will be added to exonum/src/lib.rs?

pub mod api;
pub mod helpers;
pub mod explorer_api;

I propose further splitting helpers into loggin and clap_fabric and making explorer_api part of api, so that eventually it would look like:

pub mod iron_api;
pub mod clap_fabric;
pub mod logging;

I assume, this issue has to be coordinated with #61.