Stockholm

* Updated README with examples about `units` and `nanos` properties.
* Added `get_currency(ticker)` method available directly from root package, to make it importable with `from stockholm import get_currency`.

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* Adds method `to_sub_units()` on monetary amounts which for most currencies multiplies the value with 100, but for example for `stockholm.currency.JPY` wouldn't change the value at all.
* Adds method `to_currency(currency)` (and shorted `to(currency)`) on monetary amounts which just changes the currency of the monetary amount without any kind of changes to the amount itself.

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* `Money.from_sub_units(amount, currency)` method is a classmethod which does the same thing as `Money(amount, currency, from_sub_units=True)`. For example a `100` subunits of `stockholm.Currency.SEK` would result in a monetary amount of `1.00 SEK`, while 100 subunits of `stockholm.Currency.JPY` would result in `100 JPY`.

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* Replaces `is_cents` with `from_sub_units` which is aware of number of digits in a `BaseCurrency`.

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* Added support for custom currency types with non-uppercase ticker symbols.

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* Restructured currency to also be able to fetch currencies directly on `stockholm.Currency`

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