* Beginning to add a POW description, and some minor changes to mining testing, addition of further debug information to mining output.
* Many additions to create first draft of POW documentation
* Fixes to difficult adjustments by adding a MINIMUM_DIFFICULTY consensus value. Otherwise never adjusted above 1 due to flooring.
Integrate the new difficulty calculation into the block chain
validation, the miner and tests. As the difficulty calculation
doesn't use varying Cuckoo sizes anymore and we narrowed down
reasonable final Cuckoo Cycle parameters, removed all Cuckoo
Cycle sizes from block headers.
Formalized easier Cuckoo Cycle sizes for testing (and possibly
testnet) by introducing a test mode in configuration. Updated
all tests.
* core: cleanup slicing impls for Hash
* core: clean up Readable trait, implement Readable/Writeable for various integers
* core: change Hash debug output to hex
* core: correct warnings in all modules
