don't trust Lua GC with MuPDFs objects

Use explicit fz_free_.../fz_drop_... for MuPDF objects instead
of registering a function that would do so upon garbage collection.
Garbage collection can't really be relied upon - especially, since
Lua doesn't really know how much memory the MuPDF objects claim
and would only use the footprint of a pointer for calculation.
Merge pull request #267 from hwhw/master

don't trust Lua GC with MuPDFs objects