Account class: basic staff

[ngiachou]

We need an account class that keeps information about the transactions that happen over time, as well as the current balance of the user. To achieve that we need the following basic attributes:

• balance (a floating point number representing the current user's balance)
• transactions_per_date (a dictionary with key a date string in ISO format and as a value a list of transaction objects)

We should also use some basic methods like:

• change_balance(amount)
• new_transaction() (adds a new transaction object to the transactions_per_date dictionary)

[ngiachou]

Closing since PR #26 was merged.