Replaced PartitionTextFileWriter with PartitioningDataStoreWriter - a mo...

...re generic implementation with pluggable entity writers and serialization formats.

Refactored the code so that writing, sharding and partitioning are separate concerns. 

The ShardedDataStoreWriter accepts a DataStoreWriterFactory to decouple it from the writing implementation.

The PartitioningDataStoreWriter accepts a ShardedDataStoreWriter, a PartitionStrategy and a Serializer which converts the entities to be written to the type accepted by the DataStoreWriters.

Some of the various *ObjectSupport methods have been made public so that they can be used from the DataStoreWriterFactory implementations (such as TextFileWriterFactory).
```
$ grep -R Serializer spring-hadoop-store/src/main/
spring-hadoop-store/src/main/java/org/springframework/data/hadoop/store/Serializer.java:public interface Serializer<E, S> {
```
Are you sure you've pushed all you changes to your repo? There's only the interface and nothing more.