Added error code for authentication errors.
Fixed wrong error code assignment.
Properly convert Qt network error codes to string.
Only parse the response if no network error occurred.

Parsing the response might generate errors itself and overwrite the
error information already available in qRestResult.
Added raw headers from the http response to qRestResult.

This is especially useful for retrieving Set-Cookie headers which
may contain session information like an id and expiration date.
Create result objects based on a string id using the Qt meta type system.
Removed the "result" methods which instantiate objects.
Removed template method declarations for already removed implementations.
Merge pull request #3 from saschazelzer/backwards-compatible-fixes-and-features

Backwards compatible fixes and features