higher-level API for PCF8591 that is similar to the digital GPIO pins API
higher level API for analogue output
made pcf8591read example use new API, extracted a constant for default PCF8591 address
loopback test runs against new API, deleted old loopback example
Merge branch 'master' of github.com:npryce/quick2wire-python-api into pcf8951-api
some renames and some docstrings
Minor readability improvement
fixed typo's, made pcf8591 example work with new API
Merge pull request #25 from npryce/pcf8951-api

The PCF8591 API matches our other APIs for GPIO and MCP23017