Fix for #62, #63, #64: automatically detect schema URL

[cweiske]

This patches add automatic schema URL detection to bin/validate-json.

Schema is detected from:

• "$schema" property in the root object
• "profile" property in the Content-Type header (once Validate URLs #61 has been implemented, see http://json-schema.org/latest/json-schema-core.html#anchor34)
• "describedBy" link header (once Validate URLs #61 has been implemented, see http://json-schema.org/latest/json-schema-core.html#anchor35)

This pull requests builds on:

• resolve relative uris and anchored schema uris #70
• limit recursive expansion to 7 levels #71
• show path info in invalid-schema exception #72
• Ignore mime type on json-schema.org #73
• Cache schemas #74

[cweiske]

rebased against master, so it will apply cleanly now.

[justinrainbow]

Great, thanks once again for your PR. I'll have a new version tagged later today.