Update embed_video_tags.py

Now is possible set relative sizes in template tag. Ex: '100%x500' or '500x100%' or '100%x100%'
Change the regex to allow only numbers
Add tests for relative size
Simplify regex for relative size
Allow x surrounded by spaces
Add example of relative size
Merge branch 'relative-size'

Conflicts:
	embed_video/backends.py
Merge branch 'master' of github.com:yetty/django-embed-video
Add relative size to changelog. Closes #19.