Expose list of currency pairs available for backtesting/trading

[lacabra]

Expose list of currency pairs in Catalyst so that a user can query and obtain that information by exchange.

[lacabra]

While this remains an open issue, in the interim a list of available currency pairs for backtesting has been documented on our wiki: https://github.com/enigmampc/catalyst-docs/wiki/Currency-pairs-available

[lacabra]

Fixed in commit 2ade298 (release 0.3). When trying to load a symbol that is not found (you can enter any string to trigger it), Catalyst will return a list of symbols available on that particular exchange. Also, the full list can now be accessed here: https://www.enigma.co/catalyst/status