Fix issue 157: Return different exit codes.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@592 812d1f90-f944-d88a-411a-122bdc93e1ea
Update README.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@593 812d1f90-f944-d88a-411a-122bdc93e1ea