Fixed build on OS X and bundled LMDB

Hi, nice work!

I've fixed the build on OS X and also bundled LMDB in the source tree. npm doesn't work well with git submodules, so including the entire lmdb tree in the repo is the correct approach.

I've included lmdb in the node-gyp build rather than using its Makefile, which would defeat the point in using node-gyp, and opens the door to Windows compatability. It also simplifies things as there's now no need for linking.