Changed Bitfinex' timestamps from float to BigDecimal

This pull request addresses issue #726 (also discussed in #729). Changed all timestamps from float to BigDecimal since float cannot represent Bitfinex' timestamps precisely.
Created unit tests for BitfinexAdapters and extended BitfinexTradeDataJSONTest to test the handling of json responses.

FYI: I ignored the test BitfinexTradeDataJSONTest.testPlaceOrder() since the related json response leads to an exception. The field order_id cannot be mapped to the current BitfinexOrderStatusResponse. This may be addressed in a separate issue.