generalize a bunch of boiler-plate type code for creating integers

[vtjnash]

This generalizes a lot of code in int.jl, with the goal of making it easier to create new bitstypes. While I realize immutable types have rather stolen the show, bitstypes are sometimes still nice. This reduces the number of boilerplate methods necessary to create a minimal bits type by 24!

@JeffBezanson @StefanKarpinski @ViralBShah OK to merge?

I'm also thinking of adding some generic convert, promote_rule, and arithmetic functions so that you can just call bitstype <: Signed (or Unsigned) and everything would pretty much just work.

[StefanKarpinski]

Absolutely do not merge anything like this before 0.2!

[JeffBezanson]

I'm all for thinking about how we could make something like this work, but right now it doesn't without deeper changes. As has been pointed out, the problem is that box/unbox only apply to bitstypes. See also issue #1470

[vtjnash]

I could make a macro which does the 'basics' of making a new numerical type. What we really want here is:
f{I<:Integer::Bitstype}(x::I) = ...
Or
f{I<:Integer}(x::I::Bitstype) = ...
I think there's already an open issue for considering something like this?

[simonbyrne]

A simpler option might be to define a new abstract subtype of Integer, say BitsInteger?