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.
Changes pursuant to [feedback from Schildbach](https://github.com/bitcoinj/bitcoinj/pull/107).
Adjust definition of `COIN_VALUE`; statically import `MAX_MONEY` into test file.
Merge branch 'coinops' of https://github.com/mackler/bitcoinj into coinops