add support for breaking on function entry

I was surprised that vim-dbgp didn't support breaking on function entry already but happily it was quick enough to add.  This patch seems to work for me but feel free to consider it a starting point and suggest improvements.
I guess it works as is, but please, use different variables for cmd and arguments. So at the end you do the call like this:

    msgid = self.send_command(cmd, args)