Update $xhr service

[vojtajina]

Based on in-line discussion from http://github.com/angular/angular.js/pull/75

• parse response to json if content-type is json
• browser must provide more information (at least content type)
• parse response to json if uri path extension is .json ?
• allow handling other statuses than 200 ?

[groner]

Making a responses location header available would allow $resource to know where a new record has been POST'd to.

[alexeagle]

How about splitting this issue up? Handling HTTP 201 should be easy without changing any of the parsing code.