Changes to the `Coin` class: some convenience comparison operators;
`parseCoin()` now accepts negative values; the check for an excessive
value is moved to the constructor from `parseCoin()`; some `Coin`-type
constants broken out into one `long` one `Coin` in order to be usable
in the constructor.  Corresponding tests included.  The `BitcoinURI`
class constructor throws exception on parsing a negative amount, which
is needed now that `Coin` class accepts negative amounts.
Definition of `MIN_NONDUST_OUTPUT` moved from `Transaction` class to `NetworkParameters`.