Template parameter, and remove default to nil for domaindir

Hi, great job on the module. I've added a new template parameter which allows domains to be create with a `--template` parameter in asadmin. Also, much the same as #8, I've removed the `defaultto nil` from the domaindir parameter in the rest of the types, not just domain. I made a couple readability changes, and tried to make the readme a bit clearer.