Add CInteger trait type

[burnpanck]

I think there is a need for a CInteger trait: A trait that converts anything you throw at it into a python integer if possible, without enforcing either int or long on python 2. In essence, it should simply call int(value) for coercion, which under python 2 only produces a long where needed. A typical situation is when numpy array scalars are assigned to a trait, which will be rejected by the non-coercing traits.

[minrk]

I think coercing traits are rarely actually desirable, and we've slowly been removing their use throughout IPython.

For instance, if you have an integer trait, and you do:

hastraits.int_trait = 5.5

I think it would be very surprising to get hastraits.int_trait = 5 be the result of that assignment. I would consider it a bug that 5.0 raises a TraitError, but I think that's something that should change in Integer, rather than force the addition of CInteger, which would coerce all kinds of things, like '5' and b'\9' to integers, when they should probably raise errors.

[burnpanck]

I totally agree, explicit is better than implict and errors should never go unnoticed (or something like that).

One can probably distinguish at least three classes of numbers:

1. python integers (int and in py2 long)
2. other types that always represent integer numbers (numpy array scalars and possibly 0-dimensional arrays of integer types)
3. numeric types that may represent integer numbers, but also non-integer numbers
4. types that may represent numbers, which might be integers.

It is a matter of definition what one calls coercion and what not, but I'd definitely want to have a trait that accepts the first two classes. I do not care too much if the values are kept as is or turned into python integer object (the latter could be an option for a coercing trait). I am actually undecided towards the third class, I'd tend to let traits reject them, but a design choice for a coercing trait could certainly be to accept e.g. float objects that represents an integer object. It definitely should coerce that object into one of the first two classes.

tl,dr; I want to be able to assign numpy integers to integer traits... Maybe #245 could solve that?