Add draft changelog notes for upcoming 2.3.0 release.
Add details for query performance improvements to changelog.
Minor changelog edits.
Add a page to the documentation describing how to use the LevelDB API.
Merge pull request #1137 from wiredtiger/changelog-2.3.0

Add draft changelog notes for upcoming 2.3.0 release.
Whitespace.
Add upgrading section to docs for 2.3.0 release.
Make the LevelDB API more resilient to invalid configurations.  Also clean up after the test so that "make check" has a better chance of succeeding.
Sanity check the LevelDB configuration at build time, not runtime.
Cut WiredTiger release 2.3.0