Added to_cash methods to numerics and strings

[julescopeland]

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.

[semmons99]

@julescopeland thanks for the patch!