Added option not to parse ajax params + error callback.

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We have a scenario where we're fetching parameters such as foobar[]= (Rails arrays) from the query string. They get parsed and then encoded into a foobar%5B... which Rails doesn't parse. There's no need to encode those and I didn't find a way to prevent it.

If anyone has a better suggestion, do let me know. Otherwise this is a pretty innocent change that basically preserves the url as it was set in code.

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Added onError callback (currently plugin eats errors, such as 404). Sorry for two features here, let me know if one of those is a no go and I'll figure something out to split them up.