Updated the user guide.

Mostly re Job Polling, plus some typo corrections etc.
More minor User Guide changes.

Correcte some typos, added to a style guide section; added to the cylc history
section.
Minor CUG timezone text update.
More minor CUG text updates.
Updated the Tutorial for new log locations etc.
Merge pull request #1081 from hjoliver/cug-update

Updated the user guide.