Fixed several bugs in the camera and I2C management.

Added log informations.
Modified some logging information.
The system time is now a global variable.
ueye_camera: Fixed a bug in the acquisition thread callback.
image_buffer: Images are now structures instead of simple buffers.

The structure contains both the buffer and the acquisition time.
Adapted the changes in image_buffer to the camera interface.
Adapted the whole code for the new 'image_buffer' structure.
Fixed a bug in the image saving process.