Missing values by Sentinels

Here is a draft for missing value support by sentinels.  This is my first real go at Julia so I suspect I'm doing many things badly.  Still, hopefully the general design is sound.  More on missing bits at the end of this comment.  Work was done with @dchudz.

Sentinels represent missing values (NA) by rarely used values within the domain of a datatype.  In particular we use the following values for the following types:

* `Bool`: The integer 2
* `Float32/64`: The exponent signalling `NaN` and the mantissa 1954 (the birth year of an R creator)
* `Int32/64`: The minimum integer -2**31/63 + 1
* `Complex`: The float definition of `NA` for both real and imaginary parts

We extend arithmetic and mathematical operations to respect the new semantics of these special values.  Each operation is now accompanied by an `isNA` check.

The choice of sentinel values here agrees with the choices made by the R language and subsequently by the DyND project in Python (cc @mwiebe) ([article on R sentinels](http://www.markvanderloo.eu/yaRb/2012/07/08/representation-of-numerical-nas-in-r-and-the-1954-enigma/).)

Adopting this as a cross-language standard for missing values might be of some use.

Example
-------
```julia

julia> x = Option(1)
1

julia> x + 3.14
4.140000000000001

julia> typeof(x + 3.14)
Option{Float64}

julia> x = Array{Option{Int32}}([1, 2, NA, 4])
4-element Array{Option{Int32},1}:
 1
 2
 NA
 4

julia> x[4] = NA
NA

julia> x
4-element Array{Option{Int32},1}:
 1
 2
 NA
 NA

julia> mean(x)
NA

julia> mean(nonnull(x))
1.5
```

Issues
-----

I have no idea if this is an appropriate solution for missing values in Julia.  `DataArray` (masked arrays) and `Nullable` (explicitly boxed) are two other fine solutions in various contexts.  This is just a third.

If this is an appropriate solution then certainly my implementation could use a lot of work.  In particular

1.  Option should be a subtype of Number.  When I do this I run into a large number of dispatch ambiguities.
2.  We need to extend all elementary mathematical functions.  Is there a list in Base somewhere?  DataArray has such a list.  It'd be nice if that were moved somewhere closer to core.
3.  I haven't yet invested in learning how to properly test while developing in Julia.  As a result there are no tests.
4.  In many cases when we use `Number` we really mean `{Int32, Int64, Float32, Float64, ...}` all of the Optionable types.  `Option` does not extend to non-primitive numeric types.
5.  The singleton `NA` could use better interaction with promotion rules and such.

Also, presumably people have discussed this before.  I'm coming into this without much context.  My apologies.
`promote_rule` isn't but `promote_type` uses `typejoin(promote_rule(S,T), promote_rule(T,S))` so it is guaranteed to be symmetric, even though `promote_rule` isn't.