This was a challenge because it had to work in python2 and python3,
which have almost opposite unicode models, and I couldn't use the six
library.  I'm also not sure the solution I found would work in python3
before python3.3, because I use the u'' string prefix which I think was
initially not part of python3.

This commit also:

- Dramatically increases the number of unit tests , mostly by borrowing
  from the standard  library's unit tests for math.isclose().

- Refactors approx() into two classes, one of which handles comparing
  individual numbers (ApproxNonIterable) and another which uses the
  first to compare individual numbers or sequences of numbers.