#59 support for multiple binary stars.dat

[Selden Ball]

It would help startup performance if Celestia supported multiple binary stars.dat files. The filename could be the same since they could be in separate Addon directories.

Large numbers of stars need to be added when translating scanned photographs of clusters or when constructing globular clusters. Also this would help in constructing 3D maps of the thousands of stars of some online games and other SF environments. See http://shatters.net/forum/viewtopic.php?f=2&t=12765