Revert "Add support for SvREFCNT_dec_NN"

This reverts commit a6920ca1a68d1d64558b31ad99665f46b0f80b69, since
the SvREFCNT_dec_NN implementation is leaky. (Doesn't clean up free'd
SVs)
GH #11 - Fix warning when compiling with clang
GH #12 - Fix uninitialized flags variable being used in grok_* calls
Bump version to 3.24 and update changes for release