Add network error messages to retrieve methods

Fix build on GCC < 4.9

SSL errors can be ignored correctly now

- Use QNetworkAccessManager::Operation for the operation mode

- Add ability to update resources, not only create them
- Simple interface to update an issue
- The estimated time of an issue is now saved as an double instead of a QTime
- qtredmine::Error is now called qtredmine::RedmineError
- There is a new #define NULL_ID which is used for IDs that have not yet been set