Add support for "activate" and "sender" terminal-notifier options

There is an issue with terminal notifier and 10.9 https://github.com/alloy/terminal-notifier/issues/63

Without specifying a `sender` I don't get any notifications under OS X 10.9.1
I also added the `activate` option to define what gets _opened_ when the notifications is clicked.

I also created a setting (with default of `com.apple.Terminal`) to define the server and application to be activated via bundle identifier: `set :nc_terminal, 'foo'`

For me this gives me notifications as expected.