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.
Improved vim syntax file.
Merge pull request #1081 from hjoliver/cug-update

Updated the user guide.
Use standard vim syntax groups.
Merge pull request #1085 from hjoliver/vim-syntax-update

Improved vim syntax file.