scvim: Adding tmux support, fixing screen support, fixing sclang restart/start/kill (also in terminal multiplexers)

![tmux-ide](https://f.cloud.github.com/assets/432519/1478079/a24598a2-4666-11e3-87da-9e49c153f19f.png)
Added support for tmux AND screen.
They both can now spawn sclang output in a vertical or a horizontal split/region.
Wrote an additional function to resize the sclang output within tmux or screen from scvim:
SCresize +/- number (of rows/lines)
Makes it much more convenient to work with scvim in split mode, because you can tile it all in one screen (including the SuperCollider output), also without a tiling window manager.

I switched to using $STY and $TMUX environment variables (for screen and tmux respectively), as using the $TERM variable is completely useless and unreliable for detecting if you're running in a terminal multiplexer.

Fixed SClangRestart/SClangKill commands (they were usually leaving some processes running, which could built up to a high CPU usage, if not taken care of in the long run).

Seems to also work on MacOS with macvim!
Make sure to install the following using brew:
macvim
screen/tmux
pidof