It seems elementary-tweaks has no longer key bindings in elementary Freya. But luckily there is mode to bind keys in Freya also. You can use program named xbindkeys. Details about how to configure and use it you can find here.
We only need xbindkeys, a simple yet powerful command line tool to
bind commands to a certain key or keys combinations. The program can
be installed via terminal typing:
$ sudo apt-get install xbindkeys
After the installation, if you try to run the application, you will be
warmed to create a configuration file. As user, type:
$ touch ~/.xbindkeysrc
$ xbindkeys --defaults > ~/.xbindkeysrc
Then edit the file:
$ nano ~/.xbindkeysrc
And type, before the end section:
so it will look like this:
Then press CTRL+X to save and exit.
Of course, xbindkeys can be used to bind also different commands to
different keys. The values are written as:
state (0x8) and keycode (32)
keysyms associated with the given keycodes
To find the last two values (which, as we’ve seen, can be used
$ xbindkeys -k
then, in the blank window that’ll be open, type the key or the desired
keys combination. The result will appear in the terminal.