support for Perl 5.8

I sympathize with your position on Perl 5.8 support (#4).  However,

 - requiring 5.10 for this module is arbitrary since it doesn't use any 5.10 features
 - most people still on 5.8 are not there out of choice, and requiring 5.10 is likely to make them simply not use `Alien::LibYAML` rather than to make them upgrade their Perl (I was there rather recently in fact)
 - most issues are going to be in AB rather than `Alien::LibYAML`, any issues raised will most likely be our responsibility
 - issues that *are* raised, help us shake out bugs in AB itself
 - the AB team is committed to support 5.8 as long as the other parts of the tool chain do

I understand and respect your decision if you decide not to merge this PR.