storage: Changes sstable file extension to .ldb

While new sstable file will be created with .ldb extension; old .sst
extension still supported.

Fix issue #50.
leveldb: newRawIterator: Obey strictness option

Related to issue #50.
storage: FS.GetManifest: Also check other files; beside CURRENT file

Returns ErrMissingCurrentFile instead of os.ErrNotExist if directory
contains files that belong to the DB so LevelDB will not consider
the DB is missing and overwrite the directory.

Related to issue #49.
opt: Fix comment on ErrorIfMissing option