Merge pull request #251 from sortega/issue/39_serial_exchanges

Fix how exchange amounts are computed
Split `Balance` into two different types

`BitcoinBalance` and `FiatBalance`, since they must provide different fields.
A common `Balance` trait is maintained to treat them polymorphically.
Add a `blocked` field to `BitcoinBalance`

Indicating the amount that is currently blocked in the wallet. This is also
reported in the wallet view of the UI.
Promote Spread to a proper case class
Add `minOutput` to `BitcoinBalance`

This value indicates the minimum output available in the wallet.
Split balance amount in estimated and available
Move convenience mutators of `Balance`

To the test class where it's used.
Merge pull request #253 from apoloval/task/balance-details

Improve the details of Bitcoin balance
Merge pull request #252 from sortega/issue/81_server_matching

Promote Spread to a proper case class
Add type aliases for variations on Position[T, C]
Add OrderBook.updateUserPositions

This method updates the positions of a given user by adding, updating or
removing them as necessary.
Merge pull request #254 from sortega/update_user_positions

Refactor: move user positions update to OrderBook
Fix broken test in Windows
Prevent issues with old messages in `OkPayProcessorActorTest`

We are using the same test probe to send and receive the responses of the
different test cases. If some test case finishes but the actor under test is
not terminated, it could send messages to the test probe that would be received
while having different expectations in another test case.

This commit fixes this heisenbug (CF-86) by using a test probe in the fixture,
which ensures that the requester is different from one test case to another.
Prevent issues with old messages in `BlockchainActorTest`

The same heisenbug consisting on reusing the same test probe for different
test cases.
Improve error reporting for OKPayProfileExtractor
Merge pull request #257 from edrevo/improve-okpay-error-reporting

Improve error reporting for OKPayProfileExtractor
Merge pull request #256 from apoloval/bug/cf-86

Prevent issues with old messages in `OkPayProcessorActorTest`
Merge pull request #255 from edrevo/fix-broken-test

Fix broken test in Windows
Extract brokering logic from BrokerActor to Broker