Use AppConfig.ready to register signal handlers

Django 1.6 is still supported by manually calling `register_signal_handlers` as it was before, but for Django 1.7 this can be done automatically.

Tests in 1.6 will likely not pass now, as the signal handlers are no longer registered manually by the tests. Is 1.6 compatibility still important? If so, I will fix the tests. The project template states Django `>=1.7` as a requirement, but  `install_requires` in `setup.py` states `>=1.6.2`.

References to `admin.autodiscover()` in the project template and docs have been removed as well, as this is called from the AdminConfig when `django.contrib.admin` is loaded in 1.7.
The module name should be ``wagtail.contrib.wagtailfrontendcache``