Add leveldb

Intro:
LevelDB is a fast key-value storage library written at Google that
provides an ordered mapping from string keys to string values.

URL: https://code.google.com/p/leveldb/

Howto:
The following will start the db_bench provided by leveldb.
$ make image=leveldb
$ scripts/run.py

LevelDB:    version 1.15
Date:       Wed Apr 16 06:45:29 2014
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
------------------------------------------------
fillseq      :       8.079 micros/op;   13.7 MB/s
fillsync     :    3371.135 micros/op;    0.0 MB/s (1000 ops)
fillrandom   :      10.913 micros/op;   10.1 MB/s
overwrite    :      13.146 micros/op;    8.4 MB/s
readrandom   :      11.831 micros/op; (1000000 of 1000000 found)
readrandom   :      10.534 micros/op; (1000000 of 1000000 found)
readseq      :       0.415 micros/op;  266.4 MB/s
readreverse  :       0.718 micros/op;  154.1 MB/s
compact      : 1676298.000 micros/op;
readrandom   :       6.208 micros/op; (1000000 of 1000000 found)
readseq      :       0.366 micros/op;  302.0 MB/s
readreverse  :       0.635 micros/op;  174.2 MB/s
fill100K     :    2104.607 micros/op;   45.3 MB/s (1000 ops)
crc32c       :       5.208 micros/op;  750.1 MB/s (4K per op)
snappycomp   :       7.972 micros/op;  490.0 MB/s (output: 55.1%)
snappyuncomp :       1.552 micros/op; 2517.3 MB/s
acquireload  :       0.734 micros/op; (each op is 1000 loads)