Add AstropyBuildPy, which builds the default configuration even if build_ext doesn't run

closes #719
Prevent configuration file generation in build_py if build_ext will be run
Merge pull request #9 from astrofrog/build-py-generate-config-fix

Prevent configuration file generation in build_py if build_ext will be run
Changed default config generation to use Distribution.is_pure to know when to run
Merge pull request #753 from eteq/build-py-generate-config

Fix #719 by adding build_py config generation