First commit from fork

Command.java:
-added return code to execute
-changed signature of execute to take reference to MainActivity instead of TextView

MainActivity.java:
-added username to prompt
-changed prompt to use '$' instead of '#'
-change prompt format to mimic bash
-changed error messages to mimic bash
-added code to execute given command (if valid)
-added current working directory string (still duplicated in Command.java for now)

Cd.java:
-added basic code to change the current working directory