Adding support for custom XHR with usage of $http

[wesleycho]

I would like the ability to be able to use custom XHR with $http - in particular, I would like the ability to be able to capture the progress of a file upload using the $http service, but cannot do so with the current implementation.

[prajwalkman]

#1934

[wesleycho]

Ah whoops - there's so many open issues it's hard to tell :( .