Enable endianness conversions on q(u)int8

Enable endianness conversions on q(u)int8

Lack of support for these types is not a real issue as endian
conversions on byte-sized types are no-ops. Still, the conversions are
useful as they facilitate writing of generic code. They can also be used
explicitly as a way to document in code an endian-specific binary
format:

     uchar *data;
     quint8 tag = qFromLittleEndian<quint8>(data++);
     quint32 size = qFromLittleEndian<quint32>(data);

This commit also adds a test for functions documented in the QtEndian
header.

Change-Id: I2f6c876ce89d2adb8c03a1c8a25921d225bf6f92
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>