Snapshot support.  API:

e_leveldb:snapshot(db_ref()) -> {ok, snapshot_ref()}.
e_leveldb:snapshot_close(snapshot_ref()) -> ok.
e_leveldb:get(db_ref()|snapshot_ref(), ...) -> ...
e_leveldb:iterator(db_ref()|snapshot_ref(), ..) -> ...
e_leveldb:write(..., [{return_snapshot, true}]) -> {ok, snapshot_ref()}.
e_leveldb:write(snapshot_ref(), ...) -> {error, read_only_snapshot}.

Also added support for e_leveldb:open() options:

{paranoid_checks, boolean()}
{max_open_files, pos_integer()}
{write_buffer_size, pos_integer()}
{block_size, pos_integer()}
{block_restart_interval, pos_integer()}
{cache_size, pos_integer()}