Introduce the common.bitcoin package

It offers typedefs for the most common bitcoinj types and some goodies
such as ImmutableTransaction to help with case class design.
Use ImmutableTransaction in public messages

To make it work, tune the following details:

 * Don’t use mocks in MockHandshake at they are not serializable
 * Remove UnitTestNetworkComponent as it is redundant
 * Rename from CoinffeineUnitTestParams to CoinffeineUnitTestNetwork for
   consistency
 * Remove ImplicitSender from CoinffeineClientTest as it is redundant
 * Improve BitcoinjTest to allow for test that use the blockchain in
   sequence
 * Make UnitTest compatible with AkkaSpec
Merge pull request #188 from sortega/issues/41

Introduce a `common.bitcoin` package