Examples:
Instantiate from any type of number
40.in(:usd) => #<Exchange::Money @value=40 @currency=:usd>
-33.in(:nok) => #<Exchange::Money @value=-33 @currency=:nok>
33.333.in(:sek) => #<Exchange::Money @value=33.333 @currency=:sek>
Instantiate and immediatly convert
1.in(:usd).to(:eur) => #<Exchange::Money @value=0.79 @currency=:eur>
1.in(:nok).to(:chf) => #<Exchange::Money @value=6.55 @currency=:chf>
-3.5.in(:chf).to(:dkk) => #<Exchange::Money @value=-346.55 @currency=:huf>
Instantiate and immediatly convert at a specific time in the past
1.in(:usd).to(:eur, :at => Time.now - 86400) => #<Exchange::Money @value=0.80 @currency=:eur>
1.in(:nok).to(:chf, :at => Time.now - 3600) => #<Exchange::Money @value=6.57 @currency=:chf>
-3.5.in(:dkk).to(:huf, :at => Time.now - 172800) => #<Exchange::Money @value=-337.40 @currency=:huf>