Add FileSystemAdapter file adapter

[dtsolis]

Adapter for saving files to the server's file system (previous PR https://github.com/ParsePlatform/parse-server/pull/716/files).

Can be used while starting the parse-server

var api = new ParseServer({
  databaseURI: 'mongodb://localhost:52853/dev',
  appId: 'appId',
  masterKey: "masterKey",
  serverURL: "http://localhost:1337/parse",
  filesAdapter: new FileSystemAdapter({
    filesSubDirectory: 'path/under/files/directory'
  })
});

[facebook-github-bot]

@dtsolis updated the pull request.

[codecov-io]

Current coverage is 91.26%

Merging #1098 into master will decrease coverage by -0.66% as of 7f00fce

@@            master   #1098   diff @@
======================================
  Files           91      92     +1
  Stmts         5736    5791    +55
  Branches      1043    1054    +11
  Methods          0       0       
======================================
+ Hit           5273    5285    +12
  Partial         11      11       
- Missed         452     495    +43

Review entire Coverage Diff as of 7f00fce

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[gfosco]

@flovilmart this is okay with me, can you also take a look?

[flovilmart]

That look good to me, and it passes the factory tests.