Use json serialization of enabled_methods

See issue #56

If the methods listed in enabled_methods are unicode (for example the
unicode_literals future is enabled in settings.py where SWAGGER_SETTINGS
is defined) then mark_safe will output u'get', which is not valid
javascript.

This commit uses json to output the enabled_methods, which will always
be javascript parseable.
Merge branch 'master' of https://github.com/davidn/django-rest-swagger into develop