Added support to load multi-page images via imreadmulti API.

The mechanism is universal as BaseImageDecoder is augmented with a new function to allow for easy support of multipage decoding.
Multipage support is added to Tiff decoder, and is fully functional.

Complete tests added for both imread and imreadmulti (test images added to opencv_extra repo).

NOTES:

• The API could be alternatively named imread_multi (note the underscore). I leave this open to suggestions.
• I didn't include C API changes here as I'd like to get feedback on the interface. Specifically, should CvSeq be used for the output Mat array? Or should another approach be used?
• I didn't include 2.4 branch changes as I'd like get initial approval on the change set and API before I invest the necessary effort on 2.4 branch.