Make it possible for from_diff to support custom types (issue #107)

[clj]

This makes it possible to pass a custom dumps implementation to from_diff to support types not natively supported by Python's json module.

I have not dealt with from_string and to_string which also use the json module. The easy solution here would be to deprecate them and rely on people using whichever implementation of json dumping and loading they prefer.

[coveralls]

Coverage decreased (-0.07%) to 64.914% when pulling 29c989e on paperlessreceipts:custom-types into 9e4d423 on stefankoegl:master.

[stefankoegl]

Thanks for the pull request - looks great!

Could you add a simple example to the tutorial? Thanks :)

[clj]

I will do.

Do you have any opinion on what to do about from_string and to_string?

[stefankoegl]

I see no reason to deprecate those two. We could also add an optional parameter to those two functions.

[clj]

@stefankoegl sorry about the delay in getting the PR updated, real life intervened.

I have added some documentation as requested, and used Decimal and the simplejson package as an example, as I imagine this being the primary use case for this change.

I have also made it possible to subclass JsonPatch to change its behaviour so that it is not necessary to pass dumps/loads arguments every time the individual methods are called.

Let me know if you have any comments and I'll get things turned around quicker this time :)

[stefankoegl]

Just one little inline comment. Everything else looks great!

[Alanscut]

@stefankoegl should we merge this PR?

[stefankoegl]

Merged, thanks :)