spi: ignore build for sample codes
spi: variables/method name, err handling changes to follow idiomatic go
mcp3008: changes to use external spiBus
spi: added godoc
spi: refactoring for err handling
spi: using defer for cleanup
spi: got rid of shouldInitialize arg in spibus (redundant).
spi: removing blank line
spi: finishing touches
spi: more refactoring