FR: Fancy printing methods + fmt::Display

Being able to get a string version of the amount in a given denomination would be a nice feature :) Implementing fmt::Display is kinda crucial, though. For Display, any denomination will do.

F.e. with an 
```rust
pub enum Denomination {
    BTC = 8;
    bits = 2;
    mBTC = 5;
}
```
as input.