Docs were suggesting that accessing the response with `catch` was the same as accessing it with `then`. Split the two cases and point to the Handling Errors section. Also mention the case where a rejection callback is specified as second parameter of `then`.

Adding support for custom HTTP status code error ranges

Replaced 'instace' with 'instance'

Only accept `username` and `password` as arguments

Use an `Authorization` header instead, which is a safer choice than URL embedded identities (aka `http://user:pass@example.com`). [Chrome 19 dropped support][chromium128323] for URL embedded identities in XMLHttpRequest for security reasons.

Added documentation note about how this will overwrite any existing `Authorization` header that the user may have set.

[chromium128323]: https://code.google.com/p/chromium/issues/detail?id=128323