SPIDecoder: Add CPHA and LSB/MSB option

[dj-bauer]

Implement 2 TODOs for the SPIDecoder.

• Different CPHA/CPOL modes
• selectable msb/lsb first direction

This commit moved the sampling of a specific bit into a separate helper function sampleBit(...) as it's now called twice depending on the value of CPHA.
As the state machine already tracks of falling and rising edges it was just necessary to sample depending on CPHA.

The MSB/LSB implementation was rather trivial as it expands on the existing byte modification.

if(endian == 0) { // LSB
    if(cur_data)
        current_byte = (1 << bitcount) | current_byte;
} else { // MSB
    if(cur_data)
        current_byte = 1 | (current_byte << 1);
    else
        current_byte = (current_byte << 1);
}

[dj-bauer]

As mentioned in #172