Tracking changes

Intends to tackle #113 (closed) and #117 (closed) as proposed:

  --track PATH             Track new source references before building
  --track-except PATH      Except the given element from tracking
  --track-save             Save tracked source references 

• bst build --track no longer saves the tracked refs to elements by default
• bst build --track-save is introduced to retain that functionality
• bst build --track <element> is now used to specify elements to track recursively (i.e. bst build --track <element1>.bst --track <element2>.bst target.bst)
• bst build --track-except <element> can be used to except an element from tracking

It is currently possible to track elements not in the build pipeline using this functionality - this seems like it would be unintentional, and should therefore cause a warning or error, but is hard to check for.