add clear() 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
```