Add config of user buffer size to _config.h file

- Allow users to define the number of user payloads cached by
RF24Network
- Add DISABLE_USER_PAYLOADS option to minimize memory use when used with
RF24Ethernet etc.
Merge pull request #33 from spaniakos/Development

Intel Galileo
Fix: External data type with small payloads

- Fixes frame handling for EXTERNAL_DATA_TYPE header types, mainly used
with RF24Ethernet
Fix: Multicast relay interfering with RF24Mesh

- Modified to prevent relaying of NETWORK_POLL requests
- Fix debug line