Connection

This pull request will create new file: `diaspy/connection.py` which holds code for `Connection()` object. This new object is documented in the code but also has a manual page written. 

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Great part of `Connection()`'s code was taken from `Client()`. This is because the new object took over some parts of functionality that client was responsible for:

* logging in,
* sending requests to a pod,
* providing tokens,

This means that now `Client()` is a proper client - only managing the connection and providing nice interface to the whole thing. Also, now we are not pushing the whole client to posts and conversations while initializing them - we give them only connection (which is everything they need, after all). 

As a result we also get cleaner code and clearer distinction between the tasks of every part of the code.