Initial commit
Add basic README
Add 'Requirements' section to README
Make sure device exists before opening

Without this check, the SPI interface would create the file if it did
not already exist (assuming correct permissions) but the ensuing ioctl
operations would fail.
Add docstring for bits_per_word arg in transfer
Basic setup.py

Structured as an individual module for now.
Add /build/ to .gitignore

To ignore build artifacts
Use ioctl for read

Regular reads from the spidev handle end up reading too many bytes due
to read-ahead buffering. Using the ioctl avoids this issue and allows
specifying per-transfer options if desired.