Basic live feedback functionality. Not perfect. The last call is never made. Committing to be able to build release version on master.
Merge branch 'master' into async-thread-callback
Fixed issues #38, #18 and #14.

Now adds user $PATH to Cakebrew’s environment variables before running.
Doctor and Update now run on separate threads with live feedback to the user, just like in terminal.
Added clear button to update task. Fixed bug in which sidebar selection would vanish.
Added missing proxy and environment operators to both interface functions.
Fixed plural form of formula.
Fixed plural form of formula on MainMenu.xib.
Made proxy text field secure.
App now blocks and shows warning when user tries to perform two brew tasks at the same time.
Stopped locking UI while performing installation related tasks.