Interface definition for NpmStreams

[AndrewFalanga]

The NpmStreams header needs

struct _NpmStream {
    int handle;
    mutex m;
    int Send(uint8_t *data,
             uint32_t dataLength);
    int Recv(uint8_t *data,
             uint32_t **dataLength);
};

typedef _NpmStream NpmStream;

• Doxygen comments
• GTest for the interface