Fix #62 Optional path argument in JoinMapper

[a4tunado]

Now to get source path from the mapper routine just add **kwargs to the arguments list. Here are some examples.

@dumbo.decor.primary
def map_primary(key, value, **kwargs):
  key, value = value.strip().split('\t')
  print >> sys.stderr, key, value, kwargs['path']
  yield key, value

Or you can specify desired argument directly

@dumbo.decor.primary
def map_primary(key, value, path, **kwargs):
  key, value = value.strip().split('\t')
  print >> sys.stderr, key, value, path
  yield key, value

Callable instances are also supported

@dumbo.decor.secondary
class MapSecondary(object):
  def __call__(self, key, value, path, **kwargs):
    key, value = value.strip().split(' ')
    print >> sys.stderr, value, path
    yield key, value

And previous mapper interface is working aswell

@dumbo.decor.primary
def map_primary(key, value):
  key, value = value.strip().split('\t')
  yield key, value

This approach allows easily extend interface to pass other arguments in the future

[klbostee]

Sounds good! Will try to find some time to review and merge this soonish.