Merge pull request #209 from apoloval/task/ThatUserConfig

Allow dropping reference config items while saving
Fix CrossedOrdersTest

Use the same price for both orders as there is an issue with reserving
funds when the price is better than the limit for the buyer and
increase the amounts the peers have to avoid stalling the orders.

Also, fix the time available for performing the exchange handshake and
improve logging about blocked amounts.
Merge pull request #210 from sortega/it_fix

Fix CrossedOrdersTest
Introduce `SettingsMapping` as a way to map between config and settings
Stop parsing config in settings companion objects

And start using the `SettingsMapping` trait instead.
Facilitate settings saving in `SettingsProvider`

And the corresponding code to persist the corresponding config file in case
of `ConfigProvider`.
Merge pull request #211 from apoloval/task/ThatUserConfig

Implement a feature to persist settings on disk
Introduce a class for prices

Prices are not attached to the precision of the exchanged currencies
so this helps solving the rounding artifacts problem.
Optimize currency implicits
Merge pull request #212 from sortega/issue/45_2nd_attempt

Introduce a class for prices
Reduce currency redundancy in protobuf messages

Refactor messages to avoid repeating the currency several times per
message thus avoiding the possibility of building inconsistent messages.
Merge pull request #213 from sortega/refactor_protobuf

Reduce currency redundancy in protobuf messages