Add missing makefile for leveldb

This seems to be a fork of a coin that doesn't include the makefile for libleveldb which means that users will either have to remove it and add links to their own version of it, or download and place this makefile manually.
I suggest readding it to make it easier for beginners to compile the daemon.
Merge pull request #1 from DoctorBlue/patch-1

Add missing makefile for leveldb