Added checking for dirty working directories.

If a repo is out of date and the working directory has uncommited changes
the user is told to first commit or stash their changes before
`RepoMonitor` can update automatically.

Eventually, when automatic updating is supported, users will only be given
the update option if the working directory is clean.
Added credential passing for SVN repositories.

You can now pass SVN credentials to `add_repo` and `scan_svn_repo` in
`RepoMonitor`. These credentials will be used to authenticate the SVN
command.

Also updated `VIPThemeHelper` to pass on its SVN credentials while
installing a theme.

Credentials are not supported for git.
Adding credentials options to add_repo WP_CLI command.

User can now optionally specifify the SVN credentials when adding a repo.
VIP-izing wp-admin with fancy VIP link.