Docs, space before pi char in mode line, elisp dir strings always end in "/".

* Added more prerequisites to README: osc and dash packages and SuperCollider.
* Added an example of using sonic-pi-mode-hook to README.
* Added a space before the pi character in the mode line: "Ruby π".
* Elisp directory strings should always end with "/", so I removed the "/" from `sonic-pi-server-cmd`.