Raise an ImportError rather than call sys.exit() when astropy can't be imported from a source checkout; also add a _teardown_log function to undo Astropy's logging setup--otherwise subsequent attempts to 'import astropy' in the same session lead to strange exceptions.
Fix a typo (which never came up due to the except: pass) and add the PR number to the changelog.
Merge pull request #1269 from embray/source-checkout-annoyance

Raise an ImportError rather than call sys.exit() when astropy can't be imported from a source checkout