Adds release notes for Kibana 4.2, minor cleanup elsewhere.

[palecur]

Renames what's new page to release notes.

Rendered doc can be inspected at http://kibana-relnotes.divshot.io/releasenotes

[tbragin]

Honestly, I find it hard to decide what is a key feature and what is an improvement. I'd prefer to keep it to "enhancements" (listing them from most significant to least significant) and "bug fixes" (if we want to go there).

[palecur]

Refactored; take a look at the render to see the changes.

[tbragin]

When possible, I'd try pointing to issues instead of pull requests in all cases - it preserves the discussion history and +1s. Issues are linked to the PRs for anyone that cares about implementation.

• For app switcher, let's point to this issue: Home page and app contexts  #4218
• For dark theme, this one: Dashboard background schemes #2906

In terms of wording, I'd stay away from the word "now" - just describe what the improvement is:
Issue 2855: Configure Kibana server log levels.
Issue 4204: Plugin manager for Kibana server.
Issue 4218: Interface to switch between plugins in Kibana UI.
Issue 3270: Server status page
Issue 2245: You can now customize the descriptions shown in the visualization.
Issue 2906: Option to switch dashboards to the dark theme.
Issue 1902: Tile maps support configurable WMS-compliant map servers.

I'd also add a couple of others:
Issue #2245: Customize legends for the Filter aggregation.
PR #4894: Add advanced Settings option to map values to colors.
Issue #2874: Add advanced Settings option to change default timepicker value.
Issue #2760: Add ability to filter by clicking on a legend.
Issue #485: Add color rationalization between charts.

For bug fixes, here are a few that stick out for me that are sufficiently user facing:

• Issue Export button doesn't export everything #4902: Fixed a problem with object export logic.
• Issue Area chart + date_histogram plots extra point on DST #3694: Improved timezone handling in visualizations as a result of Olson timezone support in ES.
• Issue Highlighting does not work on search embedded in dashboard #4261: Improved highlighting on embedded searches in dashboards.
• Issue Settings "history:limit" does not have effect on previous queries #4816: Improved history limit setting to clear past history.
• Issue Buckets not showing up with range aggregation as x-axis #4670: Fixed issue with range aggregation.
• Issue Pie labels missing when splitting charts by rows #4244: Fixed issue with pie chart labels.

[palecur]

Release notes and rendered version updated.

[tbragin]

Awesome, thanks Paul - LGTM