Support multiple repos

Added a configuration option (`ckanext.repo.repos`) to define which
repos information should be displayed. Repos should be defined in the
form github_organization/repo_name. If the GitHub organization is
skipped, 'ckan' is used as default, for instance:

    ckanext.repo.repos = ckan okfn/ckanext-glasgow ckanext-spatial
Add action to expose repos info to the API