Refactor Numeric class

Method `#to_BTCStr` is unnecessary.  It is basically `#to_BTCFloat.to_s`, but it is only called once (line 8 app/services/tweet/message.rb), and when it is called, it is string interpolated.  I therefore removed the method, it's spec, and altered the single method call to the existing `#to_BTCFloat`.

Moreover, the string methods `#to_satoshis` and `#to_millibit_satoshis` are basically the equivalent of calling `#to_satoshis` and `#to_millibit_satoshis` on a float, methods you defined previously.  I therefore, simplify the method by calling `to_f.to_satoshis`, etc.