Infinite length ranges inconsistently implemented

The following is inconsistent:

``` julia
k=1:Inf
first(k)   # works
last(k)   # works
k[1]       # throws inexact error
```

In 0.5, show is also broken.  

I would argue that infinite length ranges are worth implementing: it is more convenient to do

``` julia
for k=1:Inf
   # do stuff and call break
end
```

than

``` julia
k=1
while true
  # do stuff and call break
  k+=1
end
```
