Fixes #2262. Crashed with input html plugin.
Improved bootstrap error message.
jsonschema 2.3.0 support fix #105

See https://pypi.python.org/pypi/jsonschema/2.3.0, and how it is fixed for built in jsonschema formats https://github.com/Julian/jsonschema/commit/c763ab974c36891e2b8bcb6a6c1ebb1dd84009b6
Merge branch 'master' into cli_subcommands

Conflicts:
	flexget/config_schema.py