add workspace() function that replaces Main

I believe the delay imposed by restarting julia and loading packages is causing a significant amount of pain and productivity loss. This feature should help.

Example session:

```
julia> using Winston    # takes a while

julia> plot(rand(8))    # plots

julia> clear()

julia> whos()
Base                          Module
Core                          Module
LastMain                      Module
Main                          Module
ans                           Nothing

julia> using LastMain.Winston    # takes no time

julia> plot(rand(8))  # plots

julia> Winston
LastMain.Winston
```

Note there is a hack to use `LastMain...LastMain.X` as the full name of old modules. The full implications of this are not yet clear, but this should be ok for interactive use. Note also that this approach keeps everything around and doesn't free any memory, but that should be ok until you run out :)