Add checker decorator

[luiscoms]

TODO: Fix decorate method raising missing 1 required positional argument: 'self'

Introduce checker decorator in order to fix #21

Usage:

from healthcheck import checker

@checker(name='addition-works')
def addition_works():
    if 1 + 1 == 2:
        return True, 'addition works'
    else:
        return False, 'the universe is broken'

@checker
def throws_exception():
    raise Exception()

Decorate class methods

class Foo:
    def __init__(self):
        checker(self.bar)

    def bar(self):
        return True, 'It works!'