Extract out coins into their own file

[alxwrd]

Currently, the only valid coin set is GBP. It would be good to be able to use different currencies.

The file structure could become

coinhandler/
├── __init__.py
├── coinhandler.py
├── collections.py
└── coins/
    ├── __init__.py
    ├── gbp.py
    ├── usd.py
    └── etc..

With usage being

import coinhandler
from coinhandler.coins import gbp  # Required, if not all coins will be `Coin`

__init__.py for coins/ will contain the base definition of Coin.

This is related to #2, with the added thought being it'd be nice to be able to remove a whole module.

coinhandler.Coin.remove_subcoins(gbp)