Migrate Coin, Fiat, Monetary classes to Java Money

Originally reported on Google Code with ID 614

Now that the Java Money (JSR 354) API Backport is available and should support Android
/ Java 6, it should be possible to modify the bitcoinj Monetary classes to be more
compatible with this implementation.

As a rough proposal, I suggest the following:

1) `Coin` should extend the `NumberValue` class
2) `Fiat` should implement `MonetaryAmount` (or perhaps be deprecated in favor of MonetaryAmount)
3) A MonetaryAmount subclass should be created for a Bitcoin (value, currency) pair

I realize this is not going to be a popular request so soon after the conversion to
`Coin` and while JavaMoney is so new. But I wanted to create the issue for discussion
and to raise awareness so this change might be included in a future release.

The Specification supports many important and interesting use cases (such as crypto
currency and currency exchanges) and even includes a link to bitcoinj in the appendix.

References
---------

The JavaMoney project home page:
http://javamoney.github.io

The JavaMoney specification:
https://github.com/JavaMoney/jsr354-api/blob/master/src/main/asciidoc/JavaMoneySpecification.adoc

The JavaMoney API (Java 8+)
https://github.com/JavaMoney/jsr354-api

The JavaMoney Reference Implementation (Java 8+):
https://github.com/JavaMoney/jsr354-ri

The Java Money API Backport (Java 6/7):
https://github.com/JavaMoney/jsr354-api-bp

The JavaMoney Reference Implementation Backport (Java 7):
https://github.com/JavaMoney/jsr354-ri-bp

Reported by [e-mail] on 2015-04-10 01:17:46

[msgilligan]

@stokito I’ve created a module in my bitcoinj-addons repository that a Bitcoin currency provider based on the one from javamoney-shelter. I’ve also added a prototype adapter that will wrap and adapt the Exchange interface from the extensive XChange library of Bitcoin exchange clients. See the README section

[msgilligan]

An update on this:

1. The JavaMoney work that I have done is in the https://github.com/ConsensusJ/consensusj project in the consensusj-currency and consensusj-exchange modules. Feel free to try it out and submit issues or pull-requests.
2. JavaMoney will not be bundled with the JDK as some had originally hoped.
3. Although I am the one who originally opened this issue, I now believe – partly because of (2) – that it would be better not to make bitcoinj-core dependent upon JavaMoney.
4. I do think that JavaMoney is a useful library for some applications and (via the ConsensusJ modules) can be useful in Bitcoin applications.

@schildbach I opened this issue on Google Code, but since Mike imported it to GitHub, I can’t close it. I recommend we close it.