Eliminate iBeacon::data[]

[SodaqMoja]

This char buffer is only used to build up an iBeacon frame and then passed on to startImmediateBeacon.
Having the char array declared in the class reserves 50 bytes for the whole lifetime of the program. That's a waste.

I'd propose to remove it and to declare a (stack) char buffer in iBeacon::begin().

The size of the buffer (IBEACON_FRAME_SIZE) should be done with # ifndef / # define / # endif, so that it can be overruled if there is a need to.