* Support all file metadata and implement fileExists

Add support for the missing metadata from the `getFile()` method:
uploadTimestamp, action and bucketId.

Optimize file list retrieval (`listFiles()`) to avoid fetching information
about _all_ the files to get information about a single one. `listFiles`
now support an optional parameter, `FileName`, which will only retrieve
information about that file if it exists. This speeds up the call to
`getFileIdFromBucketAndFileName`.

* Code style fix

* Add metadata to example response test as well