Converted jemalloc to submodule

[paviad]

This PR converts jemalloc into a submodule which prevents code copying and enables rebasing on jemalloc/jemalloc and being able to benefit from bug fixes on that repo.

This PR is not to be merged blindly, it removes the jemalloc copy and replaces it with a submodule. The submodule currently points to my own fork of tporadowski/jemalloc --> paviad/jemalloc and points to a commit which is based on jemalloc master branch and brings it to the same state as it was in the original tporadowski/redis/develop branch.

So if you want to use this PR, you should first create a redis branch on tporadowski/jemalloc apply the 4 commits from the redis branch of paviad/jemalloc and then add it as a submodule.

[tporadowski]

Thanks, will have a look at it! I was once thinking of using submodules, but have no experience with them in Git and as far as I remember - they're not as easy as Subversion's svn:externals ;)