Add currency pair

Adds a CurrencyPair object to represent a pair of currencies. 

example

```ruby

AUDNZD = Money::CurrencyPair.new(:aud, :nzd) # <Money::CurrencyPair base: AUD, counter :NZD>

AUD = Money::Currency.new(:aud)
NZD = Money::Currency.new(:nzd)

AUDNZD = Money::CurrencyPair.new(AUD, NZD) # <Money::CurrencyPair base: AUD, counter :NZD>

# Currency#/ method
AUDNZD = AUD/NZD # <Money::CurrencyPair base: AUD, counter :NZD>

# wrap method
AUDNZD = Money::CurrencyPair.wrap('AUDNZD')  # <Money::CurrencyPair base: AUD, counter :NZD>

AUDNZD.to_s # "AUDNZD"
AUDNZD.base # <Money::Currency id: :aud>
AUDNZD.inverse.to_s # "NZDAUD"
```

This is a starting point, I think at some point the Exchanges use representations of currency pair rates. Interested in feedback!