Add __bool__() / __nonzero__() method to python rpmmi objects (ticket #153)

Add __bool__() / __nonzero__() method to python rpmmi objects (ticket #153)
- Objects supporting __len__() use (len > 0) for boolean representation,
  which normally makes sense but as the match iterator count is often
  zero despite the iterator actually existing and returning something,
  and breaks existing code (rpmlint at least)
- Adding a __bool__() (known as __nonzero__() in Python < 3) method
  returning true for non-NULL iterator fixes this and gives more
  meaningful answers than pre 4.8.0 which simply always returned True
(cherry picked from commit 40f788a7bf3741f9c613ff302d4e1b0ceec2658c)