Merge pull request #6 from ilmari/more-auto-prereqs

Add OS prereqs for GraphViz and XML::LibXML
auto option
cope with Build.PL in auto option
support -extra build tag added by travis, enabling shrplib and ithreads

Also update shrplib tag to include ithreads to match travis.
point URLs to organization repos
clarify comment in local::lib list
in progress blog post
fix typo
formatting improvements on blog post
in prove helper, skip old prove lacking -j option
refactor prove helper
Look harder for missing prereqs