Provide possibility to extend sure. Refs #31

[timofurrer]

Note: work in progress

This PR contains the following features:

• Add decorator to extend sure with a custom assertion method
• Add decorator to extend sure with a chain method
• Add decorator to extend sure with a chain property

Other Todos:

• Enforce beauty of assertion error message. How? Provide specific Exception: class Assertion(AssertionError): pass which takes expected and given arguments? Is it just best practice?
• Enforce negative check. How? Is it just best practice?
• Documentation / Tutorial

Custom assertion example

from sure import assertion


class Response(object):
    def __init__(self, return_code):
        self.return_code = return_code


@assertion
def return_code(self, return_code):
    if self.negative:
        msg = "Expected was a return code different from {0}."
        assert return_code != self.obj.return_code, msg.format(return_code)
    else:
        msg = "Expected return code is: {0}\nGiven return code was: {1}"
        assert return_code == self.obj.return_code, msg.format(return_code, 
            self.obj.return_code)

Response(200).should.have.return_code(200)
Response(200).shouldnt.have.return_code(201)

@gabrielfalcao please provide feedback regarding API. See first test for entire example usage.

Refs: #31

[timofurrer]

@gabrielfalcao Would you prefer to pass important values to the assertion function directly?

The interface would then look like:

@assertion
def return_code(self, what, is_negative, return_code):
    pass

This would have the benefit that the user does not need to know the AssertionBuilder class that well. The user would get an impression what's important for the assertion from the methods interface.

However, with that the user is kinda limited to those values - even worse would be if the user would not need the passed values but instead needs to access something from self: every linter would complain about unused arguments.

[timofurrer]

Maybe don't use setattr but registration of assertion into a dict and then use __getattr__ ...?

[timofurrer]

@gabrielfalcao what do you think about adding a which or having to the list of POSITIVES for the monkey patched PyObject properties?

[gabrielfalcao]

This developer experience is fantastic.