Capitalise spi

Capitalise spi to SPI which is more readable. It is also clear that it
is an acronym.
Refactor SPI.py

Refactor __init__
Extract several static methods for easier testing
Improved layout of file (automatic PEP-8)
Add a few tests for SPI.py
Update README.rst
Fix imports

I hope
Fix another broken import
Added build status indicator
Improved tests for SPI class
Fix imports

Original imports didn’t work correctly due to a problem in my IDE. Now,
they do.
Add tests and documentation for spi-module

Added additional tests to make sure that data is sent correctly. Also
added documentation.
Merge branch 'tests-fixing'
Merge branch 'master' of https://github.com/marcofinalist/weathervane