Cart: don't blow if gross value for cart is 0
Merge pull request #79 from iZettle/bugfix-zero-grossvalue

Cart: don't blow if gross value for cart is 0
[maven-release-plugin] prepare release izettle-toolbox-1.0.21
[maven-release-plugin] prepare for next development iteration
Messaging: caller specified message serializer

Open up the publisher service for caller specified
message serializer. The purpose of doing this is that
the caller may have custom written modules that the caller
whish to use when serializing messages to be published.

Changes:
* Rename the old MessageSerializer to DefaultMessageSerializer
 as it only consist of defaults in terms of serialization
 configuration.
* Add interface MessageSerializer which specify the contract
  for the caller to obey when implementing custom message serializers.
* Update publisher service test.
Messaging: cleanup of PublisherService

* Remove code duplication in PublisherService
Messaging: cleanup javadoc in MessageSerializer
Messaging: caller specified serializer in QueueServiceSender

* Open up for the caller to specify the MessageSerializer in
 QueueServiceSender. The purpose of this is that the caller may
 have custom written serializers/encryption modules or simply
 have configured the serializer to work in any special way.
* Do some cleanup of the QueueServiceSender.