Pail can't have relative paths

[brugidou]

I encountered this issue while attempting to use the TimeSliceStore with a relative path for local debugging.

TimeSliceStore.create("relative-dir");

calls Pail.create() and Pail.getSpecAndRoot() at https://github.com/nathanmarz/dfs-datastores/blob/master/src/jvm/backtype/hadoop/pail/Pail.java#L187 gives a NullRef exception because calling Path.getParent() seems to return a null when you go above the current directory.

I wish I could provide with a PR and a fix but i'm not familiar enough yet with Cascading and the Hadoop API and didn't take enough time to look into it.

Thanks for the work though :)