+ added minFeedAgeInSeconds (minimum time between feed publish)
+ added minValidAssetPrice (any feed for asset with lower price will be ignored)
+ added max_positive_diff/max_negative_diff (asset price should increase more than max_positive_diff in order to be published)
+ added exchange trust level (it multiplies the volume of the exchange.)
* changed publish_rule in the following way:
price is published if maxFeedAgeInSeconds have passed
price is published if price of any asset changes more than [-max_negative_diff, max_positive_diff]
* Errors fetching prices from any exchange are ignored unless exchange_trust_level > 0.8 (this is to prevent single small exchange from stopping all feeds)
* readability and formatting changes
- deleted unused files