Complete docs for command line flags

* Split flags into required and optional. Currently only `--master` is
  required.
* Alphabetize the list of optional flags for easier scanning.
* List current defaults for all options as defined in
  `MarathonConf.scala`.
* Add missing options “artifact_store” and “reconciliation_interval”.

Addresses part of issue #432.
Document ZooKeeper command line flags
Document flags inherited from Chaos
Mention URLs are usable for 'master' option
Merge pull request #467 from mesosphere/document-command-line-flags

Complete docs for command line flags