Fix NPE in CompositeRecordReader due to improper delegate initialization

The initial split created by the delegate in CompositeRecordReader should
provide the key/value objects used for the rest of processing. This was not
the case, which led to an NPE. Also, there was an issue in the getProgress
method.
Update documentation
Guard against divide by zero
Merge pull request #411 from kevinweil/jco/compositerr_proper_kv_init

Fix NPE in CompositeRecordReader due to improper delegate initialization