Load Data::Alias on demand to work around rt.cpan.org 82922.

Data::Alias + a string eval + threads will segfault.  This avoids the
bug for most code which doesn't use the alias parameter.  And it
avoids loading a module unnecessarily saving memory and compile time.

I'd leave it in even after 82922 is fixed.
Only run the test if perl is threaded.
Merge pull request #62 from schwern/bug/rt.cpan.org-82922

Load Data::Alias on demand to work around rt.cpan.org 82922.