Added fix for configuration file causing exception when importing astropy from source.
Fixed astropy/__init__.py to better clean the namespace
Changed error when default configuration file is missing to raise ConfigurationDefaultMissingError instead of a generic ValueError
Updated default config file installation to issue a particular warning class if the default is missing on astropy import
Merge pull request #655 from eteq/configuration-fix-src-import

Fix configuration generation to not crash source import