Subclass support for handlers + decorator syntax for register()

[aldanor]

This PR adds two features, both being improvements to the jsonpickle.handlers.register function:

1. An optional keyword argument base, disabled by default. If set to True, the handler will be invoked for all subclasses of the given class. If a class has a personal handler that was set explicitly, it will always take precedence (see tests).
2. Optional decorator syntax:

@jsonpickle.handlers.register(Foo, base=True)
class FooHandler(jsonpickle.handlers.BaseHandler):
    pass

// there are also some minor pep8 fixes thoughout.

[marcintustin]

Awesome. A docstring would be warranted now this is a little more complex.

[marcintustin]

This is ace!

Unfortunately, I don't have commit access, so @davvid will have to handle it.

One thing you can do to make it smoother is to ensure that travis-ci is enabled for your fork, and that all tests pass.

[aldanor]

No idea how to enable travis building the fork.

Btw: why don't you guys use something like tox to simplify local testing vs different interpreter versions?

[marcintustin]

@aldanor https://travis-ci.org/ It's pretty self-explanatory.

I'm not sure I see the advantages of tox, given the features of travis as a continuous integration tool.

[aldanor]

Ok I've pushed some minor changes and enabled the Travis build; looks all green.

Re: tox, you can run tox locally with a single keypress and ensure the tests pass on all possible interpreter versions (and dependency package versions, if need be); many major Python packages make use of that (e.g. pandas, numpy, flask just to name a few off top of my head).

Basically, this simplifies the process of testing things thoroughly before you push them, and lets you test the package exactly as it would be tested remotely on travis.

[aldanor]

Thanks @davvid 👍