Create factory for MWHttpRequest

Create factory for MWHttpRequest

This will allow classes that need MWHttpRequest to inject HttpRequestFactory
and thus make it overridable and testable.

Also made MWHttpRequest abstract class since it doesn't implement
execute anyway. Maybe a good idea to move execute to an abstract
method?

Change-Id: I5c0e035542ff5f791a21a95ed13bed4cea6906d0