Refactor price_info (#6003)

* refactor PriceInfo to use Fetch and reuse the client

* forget Fetch future to keep it running in the background

* update Debug message for price_info::Client

* wrap underlying errors in price_info client

* use debug_struct in price_info client debug implementation

* use global fetch service in price_info client

* rename gas_pricer parameter in RunCmd

* move price_info to its own crate

* fix price_info tests

* replace rustc_serialize with serde_json in price_info

* add documentation for price_info

* remove unused rustc-serialize dependency from ethcore

* fix price_info formatting

* re-export fetch crate in price_info

* remove unused cfg attributes in price_info

* add tests for price_info