Use JSONschema to validate

Currently validates:
- Talk series SVG generator
Provide a more useful JSONschema error, and update talk series schema.
Decoupled argument parsing to use argparse library
Bring argparse into talk series SVG generator.

Based on work by @haoxuany in #9.

It plays nicely with the JSONschema validator from @scgruber in #7.
Add JSONschema validation and argparse to talk series ICS and PHP generators.

Based on work by @scgruber in #7 and @haoxuany in #9.  Closes #9.
Add JSONschema for overview poster.

Also, rename "type" property to "name" for meetings in overview to avoid
confusion with the "type" property used in JSONschemas.
Bring JSONschema validation into overview SVG generator.

Closes #5, #7.
Specify minimum jsonschema version; minor change to messages.

Ensures access to draft-zyp-json-schema-04 specification.