Allow step and job execution context to be retrieved over the rest.

* This pull request contains the changes for allowing the step and job execution context to be retrieved over the rest. 
* This pull request also contains a fix to SimpleJobServiceFactoryBean for injecting the custom implementation of ExecutionContextSerializer (defaults to XStreamExecutionContextStringSerializer) which fixes the deserialization problem when execution context is deserialized using the simpleJobService. As there was no serializer set for simpleJobService, ExecutionContextDao was created without any serializer and was resulting in error logs like 
```
Cannot load execution context for job execution:
```
Here's the JIRA ticket for the same:
https://jira.springsource.org/browse/BATCHADM-179