Complement type, possible approach to #1032.

Paired on this with @punkrockpolly, see:

https://github.com/punkrockpolly/Playing-with-Julia/blob/master/negatedindex.jl

The premise of the Complement type is that it abstracts the
complement of a collection – primarily  that

	`x in c` <=> `!(x in c.collection)`

This commit punts on indexing for more than two dimensions because
that turns out to be an incredibly invasive change to huge amounts
the multidimensional array indexing code.

base/collections.jl

@@ -1,7 +1,7 @@
 
 module Collections
 
-import Base: setindex!, done, get, haskey, isempty, length, next, getindex, start
+import Base: setindex!, done, get, haskey, isempty, length, next, getindex, start, in
 import ..Order: Forward, Ordering, lt
 
 export
@@ -274,5 +274,25 @@ function next(pq::PriorityQueue, i)
 end
 
 
-end # module Collections
+# the complement of an arbitrary collection
+
+immutable Complement{T}
+    collection::T
+end
+
+in(x,c::Complement) = !in(x,c.collection)
+
+!(a::AbstractArray) = Complement(a)
+!(x::Number) = Complement(x)
 
+getindex(v::AbstractArray, c::Complement) = v[filter(i->in(i,c),1:length(v))]
+
+getindex(m::AbstractMatrix, I::Complement, J::Complement) =
+    m[filter(i->in(i,I),1:size(m,1)),filter(j->in(j,J),1:size(m,2))]
+getindex(m::AbstractMatrix, I::Complement, J...) =
+    m[filter(i->in(i,I),1:size(m,1)),J]
+getindex(m::AbstractMatrix, I, J::Complement) =
+    m[I,filter(j->in(j,J),1:size(m,2))]
+
+
+end # module Collections