Check if EMBEDLY_KEY has been set

This will check if EMBEDLY_KEY has been set in settings. If not
it will return a nice error message instead of throwing a 500
error.
Modified embeds.py to make it work

The Embedly API was not working at all because the return
value of Embedly (client) offered the attributes in dictionary
style and not as an object. Now everything seems to be fine.
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/spapas/wagtail into spapas-master