Subscribe to events with callback

[YerimB]

What I did

Made able to subscribe to a contract's event and setting a callback on it.
Also created a way to retrieve events from previously mined blocks.

Related issue: #1162 #327

How I did it

Using the network.alert.Alert class and the web3.py ContractEvents & ContractEvent class.
Created a ContractEvents class inheririting from the web3.py class of the same name.
This class adds two "public" functions to the web3.py one :

• subscribe : to subscribe to an event using an Alert (listening could be probably be improved with asyncio as mentionned in
  the Add ability to listen for events with asyncio #1421 issue)
• get_sequence : to retrieve events between two blocks.

How to verify it

I created some tests but should probably write more (taking any test idea if you got some).
If you notice anything please tell me, I would very much like to improve the way it works.

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation
• I have added an entry to the changelog

[YerimB]

Accidentally closed branch

[YerimB]

Another version that I find better than this one : here