Stylesheets should download p5subset.xml only once

[martindholmes]

@sydb and I working on Test2 have noticed that whenever a process is run that requires p5subset.xml, unless a local copy of p5subset.xml is explicitly provided, a fresh copy is downloaded even if the one it's overwriting is the same. This is pointless and time-consuming.

Using ant's get task with @usetimestamp would make this unnecessary, since it would just check with the server to see if the file has changed. This would make transformations much quicker.

[martindholmes]

@sydb and I have looked at this, and we think that this would be easier to handle if the current transformtei script were replaced by an Ant build file, so this could be more easily handled there; and in addition, such a change would make it more practical to run transformations on Windows.