pformat on pyrsistent objects

It'd be great if pprint and pformat worked "properly" on pyrsistent objects. That is, if the output of pformat(some_pmap) were comparable to pformat(some_dict).

That is, the following Python code...

from pyrsistent import pmap
from pprint import pprint

some_dict = {'foo': 'reverse the earth', 'bar': 'on a train to bristol', 'baz': 'some rather long line'}

print '## normal dict'
pprint(some_dict)
print

print '## pmap'
pprint(pmap(some_dict))

... produces this output ...

## normal dict
{'bar': 'on a train to bristol',
 'baz': 'some rather long line',
 'foo': 'reverse the earth'}

## pmap
pmap({'baz': 'some rather long line', 'foo': 'reverse the earth', 'bar': 'on a train to bristol'})

Whereas it should produce:

## normal dict
{'bar': 'on a train to bristol',
 'baz': 'some rather long line',
 'foo': 'reverse the earth'}

## pmap
pmap({
  'baz': 'some rather long line', 
  'foo': 'reverse the earth', 
  'bar': 'on a train to bristol'})

... or something similar

(Related to #69, but this is a request for enhancement, rather than a defect report. Fixing this would probably also fix #69)