Bug fixes and API extension

-Fixed bug in JUnit script that would leave .class files in
src directory after running
-Extending the API of the Pantry class
Further extended some of the classes.

-Added a valid info checker on Pantry and Ingredient to
make error checking in the Activities easier
-Added a constants class to hold any numbers that may need to be
used (ie. avoids magic numbers)
Further enhanced the JUnit test script, and wrote the JUnit test
for Ingredient. Updated README to reflect changes.