Added to_cash methods to numerics and strings

Updated readme and changelog to explain in detail, but now, things like
`123.45.to_cash`
Will return a Cash object - in most cases just a simple Cash.new(number), but for strings it's a little bit cleverer:
`'£12,345.67'.to_cash #=> Cash.new(12345.67)`
It also passes all options straight through as well
`12345.to_cash(foo: :bar) #=> Cash.new(12345, foo: :bar)`

All code is tested and passing.