XD-2286: Simple OOTB job for testing

The `timestampfile` job will append the current date/time to a file when launched. This job can be used to validate a Spring XD installation and to verify that jobs can be deployed. This serves a similar purpose to `time | log` for streams.

This job supports the following options:

#### fileName
Name of file to write timestamps to. Excludes parent directory and file extension. Defaults to the job name.

#### directory
Name of directory to write timestamp file to. Defaults to `/tmp/xd/output/`.

#### fileExtension
Extension for file to write timestamps to. Defaults to `txt`.

#### format
Format for timestamp using `SimpleDateFormat`. Defaults to `yyyy-MM-dd HH:mm:ss`.
I will modify the signature to throw `IOException`. However the interface itself defines `execute` with `throws Exception` so anyone that invokes this will have to handle the generic checked `Exception` anyway. Additionally, every implementation of this interface that I looked at also throws `Exception`.