Lazy Init Exchange Symbols From Server Instead Of Static Config

The main purpose of this initial BTER change is to propose a change to how exchange symbols are configured/created.  For API's that support it I think it would be better to rely on their call to initialize the collection of currency pairs rather that manually configuring them for each exchange.  Also I made a change to have getExchangeSymbols return a Collection instead of a List to hide the implementation choice a bit more.

see BTERBasePollingService line 64 for these changes.