Control-C exits Julia after pylab.plot()

After the wx event loop gets started, SIGINT kills the Julia repl instead of interrupting the currently executing command. To reproduce:

``` julia
julia> using PyCall

julia> @pylab

julia> pylab.plot(1);
```

At this point, if I close the plot and press ^C, Julia exits.

In the IPython source, it says that [wx resets the SIGINT handler when it's loaded](https://github.com/ipython/ipython/blob/master/IPython/lib/inputhookwx.py#L120), so I tried running `ccall(:jl_install_sigint_handler, Void, ())` the first time `app.o != pynothing` in [gui.jl](https://github.com/stevengj/PyCall.jl/blob/master/src/gui.jl#L98). This is an improvement, in that ^C now works if something is running, but something is still off, since ^C without anything running exits Julia instead of returning to the prompt.

Start with the reproduction using PyCall, @pylab, and pylab.plot(1), then read src/gui.jl around the app.o handling and the linked IPython inputhookwx.py SIGINT logic. The fix is done when closing the plot and pressing ^C interrupts active work without exiting Julia, and pressing ^C at an idle prompt returns to the prompt.

Written by the indexing model from the issue text.