[Issue #36] Make the IFO file look-up case-insensitive.

When loading the subpicture stream from a pair of SUP/IFO files, BDSup2Sub now ignores the casing of the "IFO" extension so that it doesn't display an error message about the non-existent IFO file.
Merge pull request #37 from sandrasi/master

[Issue #36] Make the IFO file look-up case-insensitive.