fixes to configuration auto-generating ported from auto-generate-config branch
_generate_all_config_items adjusted to return filename
modify astropy build to make the default configuration file
fixed unnecessarily long long in setup_helpers
implement code to install the default configuration file on import
updated documentation to reflect new behavior of on-import configuration setup
removed some debug code
added encode to subprocess communicate call to support py3.x
typo fix pointed out by @mdboom
Generation of configuration error message improvements
fixed encode/decode to making error mesages helpful in 2.x and 3.x
updated setup_helpers to run in build dir - this fixes py 3.x because before the 2to3ed version did not get run
Adjusted astropy/wcs/docstrings.py to work with configuration file builder on py 3.x
Moved ref_* scripts in coordinate accuracy tests to generate_ref_ast module.

This is necessary because the default configuration generation process visits all packages, including the test packages, so the import has to succeed even if pyast is not installed.
Merge pull request #498 from eteq/generate-config-at-install

Changes config package to generate a default config file at install and puts it in the users's astropy directory at first import of astropy