Initial stab at supporting the MAX31855/SC18IS602B combo

The intent is that this should behave the exakt same way as the 1-wire
version
Correct SPI_TC_Work timing

Should run 10 times per second if SPI was initially detected
Add debug output if SPIxfer has an invalid length

Might facilitate debugging. Also make mental note to add a timeout to
the do/while loop so we don't get stuck (forcing the wdog to kick in)
Merge pull request #29 from UnifiedEngineering/master

Refresh the max31855 branch
Bugfix SPI_TC_Init numspidevices handling

An incorrect assumption was made that SPI_TC_Work could be used during
init (before numspidevices was properly initialized). numspidevices
variable is now properly initialized before attemting to probe (and
still kept at zero until init is run).
Thanks to patrickwtlaw for tracking this thing down!
Merge pull request #30 from UnifiedEngineering/max31855-integration

Add support for the sc18is602b + max31855 combo