Added KeyModifier class, added methods to Key class, added tests in KeyModifierTest, added KeyModifierException for key modification custom exception handing, removed unnecessary imports and cleaned up comments in SymmetricEncryptionTest

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 (/////////////////////) ]