README: Improve description, remove UNOFFICIAL and EXPERIMENTAL claim

This tool may still be a collection of hack, but it is now included in
the upgrade process for Chef Server 12, and can't plausibly be labeled
unofficial.
README: Include --with-pg-config details in installation directions
README: Document common sql options

The sql_* options are valid for all current subcommands.  Save some
space by documenting them in a single location.
README: Add missing options and subcommands

This adds documentation for the many options that have been added
since the last substantial README update, as well as the `knife ec key
import` and `knife ec key export` commands.
README: Remove stale TODO section. Add a "Known Bugs" section

All of the TODOs have been completed.
Merge pull request #51 from opscode/ssd/readme-update

README: Document all current options, update description, remove stale sections