BATCH-2195: Document Spring Batch Integration

* Removed APT files
* General formatting and spelling fixes
* Converted README.md to docbook
* Expanded converted README.md docs to include usable code snippets
Removed serialisable check on ExecutionContext entries

The serialisation/deserialisation implementation is injectable, so it does not make sense to enforce the presence of this interface
Updated Spring Batch Integration to use Spring Integration 4.0.0.RC1