Revert unneeded changes introduced as a side-effect AuxPoW.
Updated version number to 1.8.
Tidy up retargeting code and debug.log output.

- remove unused int64_t retargetSpacing
- modulate timespan in an isolated int64_t nModulatedTimespan
- keep nActualTimespan intact, to retain the original value
- compress debug.log output from 5 into 3 lines while retaining
  all information.
version bump

version bump
Merge pull request #587 from rnicoll/1.8-dev-version

Updated version number to 1.8
Merge pull request #590 from sigwo/1.8-dev

version bump
Merge pull request #585 from rnicoll/1.8-dev-cleaning

Revert unneeded changes introduced as a side-effect AuxPoW.
Merge pull request #588 from patricklodder/1.8-dev-retargeting

Tidy up retargeting code and it's debug.log output