Add support for plain table format to sst_dump.

Summary:
This diff enables the command line tool `sst_dump` to work for sst files
under plain table format.  Changes include:
  * In tools/sst_dump.cc:
    - add support for plain table format
    - display prefix_extractor information when --show_properties is on
  * In table/format.cc
    - Now the table magic number of a Footer can be later initialized
      via ReadFooterFromFile().
  * In table/meta_bocks:
    - add function ReadTableMagicNumber() that reads the magic number of
      the specified file.

Minor fixes:
 - remove a duplicate #include in table/table_test.cc
 - fix a commentary typo in include/rocksdb/memtablerep.h
 - fix lint errors.

Test Plan:
Runs sst_dump with both block-based and plain-table format files with
different arguments, specifically those with --show-properties and --from.

* sample output:
  https://reviews.facebook.net/P261

Reviewers: kailiu, sdong, xjin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15903