GitHub Metadata support

[kentfredric]

This is a rebase of kentfredric/Distar-old#1

Adds a github %SPEC option visible in Makefile.PL.inc

• github user => 'Foo' assumes the distname and combines with user foo to generate a github metadata
• github path => 'Foo/RepoNameHere' forces a pre-calculated path for less lazy people
• github issues => 1, user => 'Foo', as above, but also sets up github for issues.

ENV{DISTAR_INIT_GITHUB_USER} being set during distar-init generates a Makefile.PL.inc with github values pre-populated.

ENV{DISTAR_INIT_GITHUB_USER} being true during distar-init also initialises github issues support in conjunction with the above.

[haarg]

First issue here is that using META_ADD and META_MERGE for the same fields is currently partly broken in EUMM. This was fixed in the 7.06 release, but was reverted along with everything else for 7.08/7.10. Still waiting on a new release that includes the fix. (Perl-Toolchain-Gang/ExtUtils-MakeMaker@91b7ff8)

So if the Makefile.PL used META_MERGE to specify resources, it will give bad results. This is why so far I've avoided modifying metadata in Distar.

I'm not a huge fan of forcing meta v2, but since it would only happen if you used this function it wouldn't be a huge issue. It would mean that the version would be specified differently when run on a user's machine, but META_ADD/META_MERGE are already ignored in that case.