Fetch/load multiple atoms at once

Currently atoms are being fetched one by one. There is a scald_atom_load_multiple function but it just iterates over the array of sid's and calls scald_fetch for each one. There is also an @todo tag in the function to improve it, so here is my proposal.

I added a scald_fetch_multiple, which does pretty much the same as scald_fetch, except using entity_load to load all atoms at once.