Added description in PlatformSpawn.

This is to clarify and make clear of the intent of the code.
Renamed variable.
Added comment.

To clarify the code.
Added PlatformForwardBack script.

This is to allow a platform to continuously go in one direction, and go back
to a respawn point. This will create an illusion the platform is destroyed and
instantiated again.
Added PlatformLeftRight script.

This is to allow a platform to continuously go in one direction, and go back
to a respawn point. This will create an illusion the platform is destroyed and
instantiated again.