ExtUtils::ParseXS now hosted on github

With Ken's permission, I have migrated the ExtUtils::ParseXS
repository from svn.perl.org to github.com.

In the process, I have learned many more dark secrets about svn
migration and git history rewriting than I ever wished to know.  :-)

I have:

* Cleaned up empty commit messages with a minimal message along the
lines of "Updated @files"
* Converted subversion "tag" branches to ordinary git tags and applied
a consistent (numeric literal) format

Please use this repository for any future contributions.

I will, in time, do similar conversions for ExtUtils::CBuilder and
Module::Build.  The latter will probably not happen until the release
tools are fixed up for git instead of subversion.

If there are any questions or if anyone sees any problems with the
conversion, please let me know as soon as possible.

-- David