Export-ly seems to work (mostly)

Exporting to ly seemed to work for me after being me go-to method to crash the program. Here are some things I noticed were wrong:
1.) Percussion staves need to use \drummode, not a stave in C major with a percussion clef
2.) Had weird time issues (specifically, a 5/8 bar made everything explode)

Something that could come in a later version of the export-ly is intelligent whole measure rests. Instead of using r1 r r r r r (ad nauseam), use R1*6 for any whole-measure rest

I know this isn't a priority right now, but it'd be useful for future development, I think.