Update on coho tool

[Andrew Grieve <ag...@chromium.org>]

Thought I'd summarize the state of the tool.

Usage: node ./coho command [options]

Valid commands:
>     repo-clone: Clones git repositories into the current working directory.
>     repo-update: Updates all cordova git repos in the current working
> directory.
>     list-repos: Shows a list of valid values for the --repo flag.
>     create-release-branch: Branches, updates JS, updates VERSION. Safe to
> run multiple times.
>     tag-release: Tags repos for a release.
>     audit-license-headers: Uses Apache RAT to look for missing license
> headers.
>     create-release-snapshot: Creates a signed .zip that consitutes a
> release.



The first two commands are useful outside of doing releases.
The last four are for doing releases.

I've updated the https://wiki.apache.org/cordova/CuttingReleases wiki page
instructions to include using the tool when appropriate.

I've been focusing on the repos I know most about when writing the tool, so
it will need some tweaks for other repos. For example, it doesn't do enough
updating of VERSION files on WP7 (bug CB-3543 just filed), and it probably
needs more exclusion filters for running RAT on windows/blackberry.