`diaspy/` modules refactored to use `_sessionpost()` method

The new `_sessionpost()` method covers every need for posting data and provides handy, unified interface. 

It has two obligatory arguments: `string` and `data` as in most cases only these two are used. However, it accepts two other arguments: `headers` and `params` as they are also used sometimes. 

This is better approach then calling `self.session.post()` directly. This way we can write shorter, more readable code and (if needed) process requests before sending them to the pod. 

The same idea was behind the `_sessionget()` method.

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New file added: `tox.ini` 

This is configuration file for `flake8` tool. I thought it would be good idea to have such file to avoid constant merge conflicts caused by different line length preferences. 

The default line length is set to **120**. 
> Most modern consoles can have longer lines than usual 80 (including *newline* so actually it is 79).

By default, error code **E701 is ignored**. 
> In short if oneliners it is acceptable I think. 

Feel free to adjust the `tox.ini` to your preferences (you are the maintainer) and I will pull the new file.