Enable configurable encryption of persisted storage

[hanssens]

Currently the data as it exists, either in-memory or on disk, is stored in plain-text. There should be an option to encrypt the data, thus making it more secure.

Here are the specs for this feature:

• When initialising a new instance of LocalStorage, you should be able to provide a custom encryption key for encrypting/decrypting the values.
• Encryption should be done on the level of in-memory, meaning when storing/retrieving objects. And not only when persisting it to disk.
• This added level of security by "protecting" the data comes with the tradeoff that it will affect performance on each CRU operation.
• Because of the performance hit AND it is an additional step that requires configuration (e.g. you need to provide an encryption key), it it should be configurable and disabled by default. Simplicity first.

[hanssens]

Update: implemented this in the issue4-encryption branch. Requires some testing in the field; it it blends, it can be deployed.

[hanssens]

Fixed, available in next release (v1.1.0).