use Enum for rv type

[Autoplectic]

use Enum on py3.4+ and the backported enum34 package on all others.

at the moment this would be:

class RV_mode(Enum):
names = 0
indices = 1

and each dist would grow a type which is one of these enums. Information measures would look at this property to know how to do things.

[chebee7i]

So I'm definitely +1 on distributions storing a default value for this mode that is changed when rv names are assigned. The thing I'm +0 on is whether to use Enum or not. Its certainly conceptually clearer, but I find option A below easier for users to type and understand.

some_func(d, rv_mode='names')  # A
some_func(d, rv_mode=dit.RV_mode.names) # B

Either option is a big improvement over the current boolean rv_names=True. So I'm curious to hear what others think. It seems that we get the >90% of the functional benefit of enums just by checking if the parameter is in set(['indicies', 'names']) instead. [If rv_mode is None, then we grab the default stored on the distribution.]

[fiatflux]

In most languages I'd be 100% for enum over a string label, so that problems can be caught early. But in Python typos will cause hairy runtime issues either way. So the only other consideration I can think of is high performance, which the very language is not.

tl;dr: I like string labels.

[chebee7i]

Addressed in #71.

[chebee7i]

And also in bffa2c6 from a while back.