Refactor price_info (openethereum#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

Revert " build with UI"

This reverts commit 9760ddc.

remove ethcore hub

fix error message (openethereum#6065)

bumped nanomsg

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whitespace

Backporting to beta (openethereum#5657)

* Add CHANGELOG.md (openethereum#5513)

* Add CHANGELOG.md

* Add some more verbose CHANGELOG information for beta and stable releases

* Add dates to releases

* reorg into blocks before minimum history (openethereum#5558)

* v1.6.7

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