Schema tool snaglist

Examples and pre-doc are here https://github.com/translate/pootle/wiki/Pootle-schema-util

• Running on SQLite, I'd suggest reporting that DB is unsupported in instances where it isn't able to retrieve data:

(pootle)~/dev/pootle (master)$ pootle schema 
null

• Report available tables and available apps when needed. The below would be easier if we just listed what was available to query.

(pootle)~/dev/pootle (master)$ pootle schema table
CommandError: Error: too few arguments
(pootle)~/dev/pootle (master)$ pootle schema app
CommandError: Error: too few arguments

• Use pretty print on user output to make things readable see https://docs.python.org/2.7/library/pprint.html

(pootle)~/dev/pootle (master)$ pootle schema table pootle_app_directory
{"pootle_app_directory": {"indices": null, "fields": null, "constraints": null}}