Moves build system to invoke

# Changes

Migrates the build system from using rake and paver to rake and invoke. Currently the following tasks are supported under invoke:

- assets.update
- assets.watch
- docs.build
- i18n.clean_report_dir
- i18n.dummy
- i18n.extract
- i18n.generate
- i18n.mk_i18n_report_dir
- i18n.robot_pull
- i18n.robot_push
- i18n.test
- i18n.transifex_pull
- i18n.transifex_push
- i18n.validate_gettext
- i18n.validate_transifex_config
- prereqs.install
- servers.celery
- servers.check_settings
- servers.cms
- servers.devstack
- servers.lms
- servers.run
- servers.update_db

These have been verified to work. Given the current discussion on the mailing list it is unclear this pull request should be merged whatsoever, given the incomplete status of the migration towards paver.