Add mount resource type

This PR primarily adds a mount resource type to the class, this means that an ```/etc/fstab``` entry is written for the created swap file so if a machine gets rebooted, it doesn't require a Puppet run to turn swap back on.

Also, I discovered when you ```ensure => absent```, Puppet "helpfully" filebuckets the swap file, this probably isn't useful behaviour so I stopped that from happening.

Finally, the README refers to the class as ```swap``` and with a resource type of ```swap``` so I fixed it to refer to ```class { 'swap_file': … }```.