Add a page to the documentation describing transactional durabililty.

refs #1150
whitespace
Simplify headers
Structure durability wording as 3 pages, "checkpoint durability",
"commit-level durability", and "tuning durability".
Add thread-failure wording.
document that hot backups and incremental archival can mess up with
bulk-load because it's not logged.
note there must always be at least one log file.
Wordsmith the hot backup paragraph for commit-level durability.
Merge branch 'develop' into durability-docs

Conflicts:
	src/docs/checkpoints.dox
Merge pull request #1178 from wiredtiger/durability-docs

Add a page to the documentation describing transactional durabililty.