Emailer, ls, mount_tmp, get_file

As discussed earlier, here are the changes for the emailer util.

In addition to this I've also moved some helper procs into shutit_global.

* shutit.ls - It took me a while to write a function that would just return files in a directory as a python list. Probably helpful to others?
* shutit.mount_tmp - Currently used in a couple of shutit_modules to get around the weird tmpfs issue - I think it makes sense to have this in core until the underlying issue with AUFS is fixed?
* shutit.get_file - to keep the dirtiness in one place. We've discussed this before, but I don't know what the best solution is. The one I've written copies a file in a container to some path on the host. There isn't any error handling.