Basic config support:

 - green                        # load config from $HOME/.green if it exists
 - export GREEN_CONF=zzz; green # load config from file zzz (override $HOME/.green)
 - green -c xxx                 # load config from file xxx (override $GREEN_CONF)

Config file syntax is defined by python's ConfigParser.
Import config from green
Remove singleton attribute from config.get_config() in setUp rather than tearDown
Import configparser/ConfigParser portably across python versions
Must import configparser/ConfigParser portably in config module also
Merge pull request #24 from tbarron/config

Basic config support (branch config) - contributed by tbarron