Rewrote coho tool.

Initially supported commands:
    repo-clone: Clones git repositories into the current working directory.
    repo-update: Updates all cordova git repos in the current working directory.
    create-release: Creates a signed .zip that consitutes a release.
    list-repos: Shows a list of valid values for the --repo flag.
Added repo-branch command.

Intended use is to create release branches.
[CB-3305] create-release: Clean the release directory first

Also tweaked directory names for create-release
[CB-3305] Use --no-merges when creating changelog
[CB-3305] Change repo-branch into prepare-release.

Command branches, builds JS, updates VERSION and tags.
Update usage message for prepare-release
[CB-3305] Split creating release branch and tagging release branch into separate commands

Also made several bugfixes & tweaks, as this was the first time these
commands were used for a release!
Coho - only call git rebase when there is an update.
Add running RAT to coho tool.

Tested only for iOS, so exclusion filters need updating for other repos.
Add a .gitignore for coho
Update RAT excludes for Android / mobile-spec / JS
Add a note about using create-release-branch to checkout branches.
Add blackberry VERSION file updating logic