spi: added spi feature for all bbb and rpi
spi: changes in fields/symbols to follow idiomatic go
spi: use a initializer if required by any host
mcp3008: added package and samples for mcp3008 10-bit 8-channel adc
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
spi: remove types from const declarations and added few more go docs
spi: fix for breaking build
spi: code fixes in samples
Merge pull request #6 from kunalpowar/spi

spi: feature