Add manifest_dump tool (#204)

Adding a tool to dump Titan manifest file, for easy debugging.

Sample output:
```
next_file_number: 93015
column family: 1

column family: 2

column family: 3

column family: 0
file 92978, size 44644407, level 0
file 92997, size 44513780, level 0
file 93001, size 44639151, level 0
file 93005, size 44420372, level 0
file 93009, size 44595346, level 0
file 93010, size 23167116, level 0
file 93011, size 38966307, level 0
file 93012, size 21745288, level 0
file 93014, size 44181211, level 0
```

Signed-off-by: Yi Wu <yiwu@pingcap.com>