Group collectable data into a struct rather than having different variables

[JasoonS]

Currently the each variable relating to a token is in its own mapping. It could be more efficient to make a single collectable 'struct' to store all these variables. (for a single map lookup in functions that use/modify multiple variables).

Just a thought about a potential downside, if we use ZeppelinOS for upgradability, then we won't be able to modify the shape of that struct. But we could keep on adding new mappings.