Add explanation to the TenhouRecord.game_list.events

[canuse]

The current format of element 'event', which is like 'D70', 'U128'... , is quite hard to understand. It might be more readable if there is an explanation, namely 'deal 1s', 'discard 1s'.

[Ledenel]

Now the element 'event' is just plain XML Element parsed by native Python library xml.etree.ElementTree .

for better representation, we could:

1. add a wrapper, somthing like TenhouEvent for the XML Element.
2. in TenhouEvent delegate everything to thet real XML Element.
3. add some helper function and represention like __str__ and __repr__.
4. hack this point to ensure wrap any XML Element to TenhouEvent.

[Ledenel]

Would be something like this:

[Ledenel]

implemented at #15 .