feat(CustomPersistance): Added custom persistance

[Drarig29]

This is an indirect way of adding localStorage support (#22) and more...

It's fully backward compatible and enables the user to:

• Disable the default file persistance behavior by passing null as a filename in the constructor
• Give a custom saving method by passing a saveCallback in the constructor, which can be used with saveOnPush and save()

Although there won't be any file to load data from, you can still use resetData(data) to initialize data.
And you can use getData() or getData("/") to get the full data.

[coveralls]

Coverage increased (+0.2%) to 95.053% when pulling 8e5dc8e on Drarig29:persistance into 95fc6ab on Belphemur:develop.

[Drarig29]

Actually, I did this to be able to use node-json-db also in the browser, but:

• It would be weird to use package named node-... inside a browser.
• I thought using webpack would remove all imports about node's fs and that it would work seemlessly but it's not the case...

And without this, I don't know if this PR makes much sense anymore.

[kartonrad]

Yes this PR could have been useful for encrypting the json file!