refactoring of the bridge logic to work with authorities

[akolotov]

I looked at the project swarmcity prepared: https://github.com/swarmcity/SCLabs-erc20-bridge/tree/master/contracts.

Seems that we can use great ideas for the bridge logic decomposition:

1. Since logic of working with authorities is the same for both bridge contracts introduce it in separate contract.
2. Do authorities as a map instead of an array. It will reduce gas consumption.

[akolotov]

And I will create an issue in the main project to address functionality of extendable list of authorities similar to paritytech/parity-bridge#33.

[rstormsf]

@akolotov so I started learning this story and I couldn't understand what needs to be done.