Improved README to use Markdown and provide additional instructions for people getting started
Peer: Add a version handshake future.
Support for applications recording exchange rate that was valid when transaction was sent. Use the new SendRequest.exchangeRate register.

Includes a test.
Orchid: fix infinite task creation loop when shutting down
Orchid: better thread tracking

also, fix race condition in closing circuits on shutdown
Orchid: document an issue with pending circuits on shutdown
Orchid: refactor thread pool creation
Orchid: narrower locking
Orchid: typo
Rename CoinFormat to MonetaryFormat, now that it can deal with Coin and Fiat mostly the same way.