Make Task#source default to the first prerequisite in non-rule tasks.
Make #sources a synonym for #prerequisites if @sources isn't set.

I've opted for a pretty conservative definition of this - `#sources`
will prefer to return `@sources` even if `@sources` is set to `nil`
Merge pull request #215 from avdi/source-defaults-to-first-prerequisite

Make Task#source default to the first prerequisite in non-rule tasks.