Note limitations of JSON backend
Allow to set the collection through a Meta attribute.
Merge branch 'json-limited' of github.com:cmutel/blitzdb into cmutel-json-limited
git push origin masterMerge branch 'cmutel-json-limited'
Merge branch 'master' of github.com:adewes/blitzdb

Conflicts:
	docs/source/tutorials/basics.rst