Copy DLL dependencies to output dir. Fixes #109

This is a wildcard copy that will also include all DLLs that may be added
in the future since the same problem may occur if another lib is included
that is not already installed in the system.
Make DLL copy conditional

This changes the copy command to xcopy and an opt that tells it to only
copy if the source is newer than the destination. Avoids unnecessary
overwrites. Didn't include it in the initial commit as it's slightly more
complex.
Add myself to credits.txt

Taking Agustin up on the offer while I'm at it. Didn't do so before because
libktx not coming through made me feel like it wasn't appropriate.
Merge pull request #110 from Lagg/master

Copy dlls to output directory