Fix configure script to use current QMake features directory
Improved Core and Network, see long comment

Changes include:
Core:
 - Fixed build on Windows (as in, don't compile UnixSignalHandler)
 - Introduced TemporaryBufferDevice
Network:
 - Big refactoring of HttpClient and infrastructure connected to it.
 - Implemented HttpWriter and HttpParser for HttpClient
New tests:
 - Core: TemporaryBufferDevice
 - Network: HttpClient, HttpParser and HttpWriter