Merge pull request #14 from jmaheswaran/master

Added KeyModifier, KeyModifierTest, KeyModifierException classes, updated Key and SymmetricEncryptionTest classes
Merge pull request #16 from jmaheswaran/master

moved exceptions to new package
Merge pull request #17 from jmaheswaran/master

moved keyversion
Merge pull request #18 from jmaheswaran/master

moved files
added HMAC key version, SigningException, and tests for HMAC key version
Merge remote-tracking branch 'upstream/master'