Task: abstract blockchain access away, make implementation agnostic

[ghost]

Be best way to implement both #4 Golos support and #38 try out steemdata would be to abstract blockchain access away from usage code and provide a standard interface to fetch data and request changes (e.g. cast votes) to the blockchain.

Note that these different implementation can be switched at run time so there will be no need to fork the project for Golos support. Similarly, we don't need to permanently choose between steem.js and steemdata, or any other blockchain access method for that matter.

A separate ticket should be raised to add blockchain access method changing in the UI.

[ghost]

Another possible way to access the blockchain is steemjs-lib, built from steem.js