Start the basis for parsing the spritesheet tags

basically starts to line down to creating a spritesheet vector that
will in order hold all the data for the spritesheets to be passed
on to the image.cpp for extracting the sprites.  NOT DONE!
Add comments and put structs into configs

Added approriate comments and moved the spritesheet's sprite_data
struct into a similar format to the portraits.
Add spritesheet.cpp and spritesheet.hpp

Initial commit of the spritesheet functions for the config/cfg
and its header file.
Add spritesheet.hpp to unit_animation.cpp
Remove redundant if statement

In image.cpp removed a redundant if statement in the
simplify_type function.
Merge pull request #211 from Aishiko/spritesheet

Spritesheet Initial commit

Doesn't compile but needed to push for review.