types: addition of EncryptedType

Addition of `EncryptedType` in sqlalchemy-utils types.
All these test methods could be refactored into a base class (for example EncryptedTypeTestCase). Then we wouldn't need to repeat the code in `TestFernetEncryptedType` and `TestAesEncryptedType`. Also these test methods should be named better to correlate with what they are testing. For example this test method could be named `test_unicode_encryption`.