Receive foreign-FSK format packages

Receive foreign-FSK packages form other sources then standard JeeLib
transmissions. Those packages are assumed to have a programmable fixed
length, as standard packages have their length in the third byte of the
package header.
fix merge error
Add soft-initialization function

Add rf12_restore which is a soft initialization which reprograms the
RFM12B without performing an SPI-init and a SW-reset of the RFM12B. This
allows for rapid switching between OOK and FSK and foreign-FSK and
native-FSK
fix error in comment/documentation
Merge pull request #45 from SevenW/rf12mods

Rf12mods-WS