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.

先使用 PyCall、@pylab 和 pylab.plot(1) 进行复现，然后阅读 src/gui.jl 中 app.o 处理附近的代码，以及相关的 IPython inputhookwx.py SIGINT 逻辑。当关闭绘图并按下 ^C 能中断正在进行的工作而不退出 Julia，并且在空闲提示符处按下 ^C 会返回提示符时，修复就完成了。

由索引模型根据 Issue 内容生成。