Include leveldb source code in riak source distribution

When building riak's source tarball, `git archive` is used
to ensure clean repositories are used rather than working
directories that might have uncommited changes.  This works
fine except for one special case where eleveldb downloads
leveldb into its source tree.  In this case 'git archive'
then removes the leveldb source because it is not truly
a committed file in eleveldb's repository.

The side effect of this issue are users who try to build
riak from source while either offline or without access
to github while building.  Any build will fail because
eleveldb does not have leveldb's source tree that is needed.

This commit fixes that by using a work around to `git archive`
in the one special case of eleveldb.  The `.git` directories
are still cleaned, but the leveldb source tree is left intact.

The one downside to this change is eleveldb is not treated
differently than every other repository and local changes
to eleveldb's source tree can possibly leak into the source
tarball.  This however is very unlikely due to `make dist`
using a completely separate `make deps` chain than the standard
developer `make deps`.  Basically someone would have to really
try hard to mess it up.

This addresses basho/riak#345
Merge pull request #347 from basho/jem-dist-include-leveldb

Include leveldb source code in riak source distribution