Major enhancements with batch processing

This SDK made a huge step forward and now ships with:

• Read, write, remove not only POJO models but also single key-values
• AlexaStateObject wraps single key-values read from and written to data stores
• Write multiple POJO models or single key-values to the handler to leverage batch processing capabilities of Alexa Session, DynamoDB and AWSIoT
• Read multiple single values from the handler to leverage batch processing capabilities of Alexa Session, DynamoDB and AWSIoT
• Remove multiple single values or POJO models to leverage batch processing capabilities of Alexa Session, DynamoDB, S3 and AWSIoT
• Handlers got exists methods to check presence of single values or POJO models in a store
• Improved unit tests
• Integration tests