* 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
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