Fix MonoImage refcount leak that causes images to not be released on
domain unload which causes new domains to reuse old assemblies referenced
by a leaky image, causing all kinds of funky behaviour. Callers should
call mono_image_close after calling mono_assembly_load* to ensure things
don't leak.
Merge pull request #73 from Unity-Technologies/unity-staging

Fix MonoImage refcount leak that causes images to not be released