Completed Cd Command

MainActivity.java:
-added proper command splitting (will now split the command `cd "foo bar"` into the command cd with the single parameter "foo bar" [sans quotes])
-added method to clear screen
-added listener for key presses (should execute command on enter/return)
-added scrolling so that the last output is always visible

Cd.java:
-changes directories properly, checking that the directory exists and is readable
-handles relative paths (cd .., cd /sdcard/.., etc.)

Clear.java:
-clears screen