Test using Clang address sanitizer

I recently caught a memory error in ForestDB using the Clang address sanitizer, and Tai Tran suggested using as part of ForestDB's pre-merge process.

The docs are at http://clang.llvm.org/docs/AddressSanitizer.html

It requires LLVM 3.1 or later. You mostly just have to add the compiler flag -fsanitize=address.