Added cacheCredentials config flag for saved SVN credentials to control the --no-auth-cache flag

Example config:

{
    "http-basic": {
        "svn.example.com": {
            "username": "user",
            "password": "password",
            "cacheCredentials": false
        }
    }
}
Codestyle fix
Moved config option to not save subversion credentials to repository configuration, added documentation.
Merge branch 'master' of https://github.com/composer/composer
renamed cache-credentials to svn-cache-credentials
fixed typo
Merge pull request #3272 from bashofmann/master

Added cacheCredentials config flag for saved SVN credentials