error paths for additionalItems, additionalProperties and patternProperties

[gazpachoking]

This fixes the additionalProperties error path problems issue in #74, which turns out was also affecting patternProperties and additionalItems.

This also fixes a problem with additionalItems, where it was not validating the correct items. See: gazpachoking@4a7ffc1#L0L221

[gazpachoking]

We should add a test to the suite that would have caught the second error with wrong additionalItems being validated, surprised that slipped through.

[Julian]

Haven't played with this locally yet, but I have a feeling this is at least partially not a bug. What happens when you have {"additionalProperties" : False}? jsonschema has an (unwritten but we should add it to the docs) guarantee that the path to a thing on the instance always exists. I think from glancing at this it would break that guarantee?

If so I guess I understand that we do need to solve this but we have to think about it a bit more carefully.

[gazpachoking]

In the case where additionalProperties is false, this block gets used https://github.com/gazpachoking/jsonschema/blob/4a7ffc107603e5a97462f5871f3c15a2f5430f1b/jsonschema.py#L196-L198 and nothing will be appended to the error path.

[Julian]

Ah, right. Forgot it's handled separately. Nice. OK well, if you think we can get #77 merged immediately and do a v1.0.1 with it then we can do that, otherwise feel free to merge this and then we'll merge your format exception thing and do a v1.1.0 along with #77.