Add MONGO_EXPOSE_MACROS for compiling on Fedora

Looks like this needs to be set or functions like "verify" don't get defined, and we won't compile. Not sure how this got set on Ubuntu.
Added malloc.h to metadata.h

There are places within the mongo headers that malloc is needed, and this is preventing compilation on Fedora.
Merge pull request #1 from cottsay/master

warehouse won't compile on Fedora