Added KeyModifier, KeyModifierTest, KeyModifierException classes, updated Key and SymmetricEncryptionTest classes

Added KeyModifier class - used to create and modify Key objects
added methods to Key class - used by KeyModifier to modify and update the Key
added tests in KeyModifierTest - JUnit tests, followed test driven development model and wrote tests before writing the code
added KeyModifierException for key modification custom exception handing - thrown when invalid operations are attempted through the KeyModifier class
removed unnecessary imports and cleaned up comments in SymmetricEncryptionTest - comments cleaned up according to Cory's previous suggestions [using multi-line comments instead of an entire line of forward slashes (/////////////////////) ]