* speed up Travis startup
- keep all test tempdata in target/tmp
- purge before saving target/* to S3 cache
* allow 4 minutes for cache up/download from Travis' S3 cache stores
* Adding kcov code coverage to chain and core modules
* Adjust travis.yml to wget kcov from mimblewimble org
* Add codecov badge to README.md
* Fix repo name in badge to grin, from kcov
* move fast tests towards the end, slow tests to the front. avoids having a long running test straggling behind when other tests have finished
* testing if we can manage also with "sudo: false"
* reorder tasks a bit more
* introduce grin_keychain, encapsulate derivation of secret_keys
* core compiles against keychain, tests don't run yet
* core tests are now passing against keychain
* wip - getting wallet working with keychain
* add util and keychain to travis test matrix
* basic test around key derivation
* Adding output stats from plugin, when available
* adding grin config
* moving pow crate from core/pow to it's own crate
* moving POW/mining stuff into pow module, and removing mining dependency from chain
* refactored most of mining into pow module...miner.rs still in server
* update grin tests
* updating genesis block mining to use configured miner, if available
* chain tests back into chain, done with pow refactor
* use tag of cuckoo-miner for pr
