New protocol decoder: rfm12

libsigrokdecode now supports yet another protocol decoder, rfm12.

This one decodes the SPI-based protocol of the HopeRF RFM12 and RFM12B wireless FSK transceiver modules. These devices can transmit and receive in the 433MHz, 868MHz and 915MHz bands (depending on chip).

Thanks a lot to Sławek Piotrowski for contributing the decoder (as well as an example file for sigrok-dumps)! We've also added a small test-case in the sigrok-test repository to be able to keep track of any potential regressions that might occur later.