Merge pull request #120 from norbertpotocki/doc-fixes

Documentation improvements for section 6.1
Added support for Snapshotting in DisruptorCommandBus

The Repositories in the DisruptorCommandBus now support EventStreamDecorators,
meaning it is possible to specify a snapshotter trigger. The Spring XML support
has been adapted to allow configuration of a snapshotter trigger, similar to
the 'regular' repositories.

Issue #AXON-237 Fixed
AggregateAnnotationCommandHandler now returns identifier on aggregate creation command

Instead of returning null, the callback is invoked with the identifier
of the aggregate that was created. This can help in cases where
server-generated identifiers are being used.

Issue #AXON-238 Fixed
Added @AnnotationDriven annotation for Spring Java Config

This annotation removes the need to "manually" configure the annotation
postprocessor beans when using Java Config.

Issue #AXON-240 Fixed
It is now allows to call apply() from an @EventSourcingHandler

This allows for easier event handling in more complex aggregate structures,
where events applied by a entity can be handled and cause new events in
another entity.

Issue #AXON-236 Fixed
several small fixes to chapter 7 of the reference guide
Merge pull request #121 from tomsoete/master

several small fixes in chapter 7 of the reference guide
CommandGateways can now be configured with Callbacks using the factory

This is useful when a callback needs to be invoked for each command sent. With
this mechanism, that callback doesn't have to be passed via each method
invocation on the gateway.
If the generic type of the callback is detected to be incompatible with the
actual result of a command, the callback is ignored for that result.

Issue #AXON-242 Fixed
Added duplicate key exception checks on snapshot appending

Similar to inserting domain events, appending a snapshot event when an entry
already exists with the same aggregate identifier and sequence number will
throw a ConcurrencyException.
The snapshotter catches this exception and logs a single line on info level,
instead of logging the entire stacktrace.

Issue #AXON-248 Fixed
Added class check to MethodMessageHandlerInspector

This is to make sure old instances are ignored after classloader change.

Issue #AXON-246 Fixed