eltype updates for chain(), count()

In response to @JeffBezanson's comments here: https://github.com/JuliaLang/Iterators.jl/pull/11

*Edit:* Updated the pull request to the following, which I think makes the most sense:

* `count()`: produce a consistent element type
* `chain()`: instead of `promote_type`, find common supertype of iterators

```julia
julia> eltype(count(1, 2.0))
Float64

julia> collect(take(count(1, 2.0), 4))
4-element Array{Float64,1}:
 1.0
 3.0
 5.0
 7.0

julia> eltype(chain(1:2:5, 1.0:.1:1.3))
Real

julia> collect(chain(1:2:5, 1.0:.1:1.3))
7-element Array{Real,1}:
 1  
 3  
 5  
 1.0
 1.1
 1.2
 1.3
```