fix: unexpected behavior with != operator. (fixes #26)

[vaab]

This is a proposal fix for #26, in an attempt to superseed #28 in being more up to the point and avoid reimplementation and try to be future proof and pythonic : it intends of implementing actual python 3 behavior.

[vaab]

I added tests and README.rst documentation to show that it allows the other object to implement its own equality methods.

[coveralls]

Coverage increased (+0.02%) to 99.149% when pulling 38947e0 on fix26 into 69f5add on master.